diff --git a/Source/Core/Core/Core.cpp b/Source/Core/Core/Core.cpp
index 2972760d34..14041f3d41 100644
--- a/Source/Core/Core/Core.cpp
+++ b/Source/Core/Core/Core.cpp
@@ -127,15 +127,15 @@ void SetIsFramelimiterTempDisabled(bool disable)
 }
 
 std::string GetStateFileName() { return s_state_filename; }
-void SetStateFileName(std::string val) { s_state_filename = val; }
+void SetStateFileName(const std::string& val) { s_state_filename = val; }
 
 // Display messages and return values
 
 // Formatted stop message
-std::string StopMessage(bool bMainThread, std::string Message)
+std::string StopMessage(bool main_thread, const std::string& message)
 {
 	return StringFromFormat("Stop [%s %i]\t%s\t%s",
-		bMainThread ? "Main Thread" : "Video Thread", Common::CurrentThreadId(), MemUsage().c_str(), Message.c_str());
+		main_thread ? "Main Thread" : "Video Thread", Common::CurrentThreadId(), MemUsage().c_str(), message.c_str());
 }
 
 void DisplayMessage(const std::string& message, int time_in_ms)
diff --git a/Source/Core/Core/Core.h b/Source/Core/Core/Core.h
index 8fff3efccb..b86168f3f6 100644
--- a/Source/Core/Core/Core.h
+++ b/Source/Core/Core/Core.h
@@ -40,7 +40,7 @@ enum EState
 bool Init();
 void Stop();
 
-std::string StopMessage(bool, std::string);
+std::string StopMessage(bool, const std::string&);
 
 bool IsRunning();
 bool IsRunningAndStarted(); // is running and the CPU loop has been entered
@@ -59,7 +59,7 @@ void Callback_WiimoteInterruptChannel(int _number, u16 _channelID, const void* _
 void DisplayMessage(const std::string& message, int time_in_ms);
 
 std::string GetStateFileName();
-void SetStateFileName(std::string val);
+void SetStateFileName(const std::string& val);
 
 void SetBlockStart(u32 addr);
 
diff --git a/Source/Core/Core/CoreParameter.cpp b/Source/Core/Core/CoreParameter.cpp
index d8b364893a..5bed7687f7 100644
--- a/Source/Core/Core/CoreParameter.cpp
+++ b/Source/Core/Core/CoreParameter.cpp
@@ -329,7 +329,7 @@ bool SCoreStartupParameter::AutoSetup(EBootBS2 _BootBS2)
 	return true;
 }
 
-void SCoreStartupParameter::CheckMemcardPath(std::string& memcardPath, std::string gameRegion, bool isSlotA)
+void SCoreStartupParameter::CheckMemcardPath(std::string& memcardPath, const std::string& gameRegion, bool isSlotA)
 {
 	std::string ext("." + gameRegion + ".raw");
 	if (memcardPath.empty())
diff --git a/Source/Core/Core/CoreParameter.h b/Source/Core/Core/CoreParameter.h
index cb0c81db2e..6c70277c05 100644
--- a/Source/Core/Core/CoreParameter.h
+++ b/Source/Core/Core/CoreParameter.h
@@ -254,7 +254,7 @@ struct SCoreStartupParameter
 	void LoadDefaults();
 	bool AutoSetup(EBootBS2 _BootBS2);
 	const std::string &GetUniqueID() const { return m_strUniqueID; }
-	void CheckMemcardPath(std::string& memcardPath, std::string gameRegion, bool isSlotA);
+	void CheckMemcardPath(std::string& memcardPath, const std::string& gameRegion, bool isSlotA);
 	DiscIO::IVolume::ELanguage GetCurrentLanguage(bool wii) const;
 
 	IniFile LoadDefaultGameIni() const;
diff --git a/Source/Core/Core/HW/EXI_DeviceIPL.cpp b/Source/Core/Core/HW/EXI_DeviceIPL.cpp
index abe415c3b9..1a15b7de2a 100644
--- a/Source/Core/Core/HW/EXI_DeviceIPL.cpp
+++ b/Source/Core/Core/HW/EXI_DeviceIPL.cpp
@@ -140,7 +140,7 @@ void CEXIIPL::DoState(PointerWrap &p)
 	p.Do(m_FontsLoaded);
 }
 
-void CEXIIPL::LoadFileToIPL(std::string filename, u32 offset)
+void CEXIIPL::LoadFileToIPL(const std::string& filename, u32 offset)
 {
 	File::IOFile pStream(filename, "rb");
 	if (pStream)
diff --git a/Source/Core/Core/HW/EXI_DeviceIPL.h b/Source/Core/Core/HW/EXI_DeviceIPL.h
index 49186ab2f7..b7b80b3a69 100644
--- a/Source/Core/Core/HW/EXI_DeviceIPL.h
+++ b/Source/Core/Core/HW/EXI_DeviceIPL.h
@@ -68,5 +68,5 @@ private:
 	bool IsWriteCommand() const { return !!(m_uAddress & (1 << 31)); }
 	u32 CommandRegion() const { return (m_uAddress & ~(1 << 31)) >> 8; }
 
-	void LoadFileToIPL(std::string filename, u32 offset);
+	void LoadFileToIPL(const std::string& filename, u32 offset);
 };
diff --git a/Source/Core/Core/HW/GCMemcardRaw.cpp b/Source/Core/Core/HW/GCMemcardRaw.cpp
index df2dc68d54..a5dd50e03f 100644
--- a/Source/Core/Core/HW/GCMemcardRaw.cpp
+++ b/Source/Core/Core/HW/GCMemcardRaw.cpp
@@ -15,7 +15,7 @@
 #define SIZE_TO_Mb (1024 * 8 * 16)
 #define MC_HDR_SIZE 0xA000
 
-MemoryCard::MemoryCard(std::string filename, int _card_index, u16 sizeMb)
+MemoryCard::MemoryCard(const std::string& filename, int _card_index, u16 sizeMb)
 	: MemoryCardBase(_card_index, sizeMb)
 	, m_filename(filename)
 {
diff --git a/Source/Core/Core/HW/GCMemcardRaw.h b/Source/Core/Core/HW/GCMemcardRaw.h
index b337eed440..3fb90a8602 100644
--- a/Source/Core/Core/HW/GCMemcardRaw.h
+++ b/Source/Core/Core/HW/GCMemcardRaw.h
@@ -17,7 +17,7 @@ class PointerWrap;
 class MemoryCard : public MemoryCardBase
 {
 public:
-	MemoryCard(std::string filename, int _card_index, u16 sizeMb = MemCard2043Mb);
+	MemoryCard(const std::string& filename, int _card_index, u16 sizeMb = MemCard2043Mb);
 	~MemoryCard();
 	void FlushThread();
 	void MakeDirty();
diff --git a/Source/Core/Core/IPC_HLE/WII_Socket.cpp b/Source/Core/Core/IPC_HLE/WII_Socket.cpp
index ca8f457738..928e59fff1 100644
--- a/Source/Core/Core/IPC_HLE/WII_Socket.cpp
+++ b/Source/Core/Core/IPC_HLE/WII_Socket.cpp
@@ -77,7 +77,7 @@ static s32 TranslateErrorCode(s32 native_error, bool isRW)
 	}
 }
 
-s32 WiiSockMan::GetNetErrorCode(s32 ret, std::string caller, bool isRW)
+s32 WiiSockMan::GetNetErrorCode(s32 ret, const std::string& caller, bool isRW)
 {
 #ifdef _WIN32
 	s32 errorCode = WSAGetLastError();
diff --git a/Source/Core/Core/IPC_HLE/WII_Socket.h b/Source/Core/Core/IPC_HLE/WII_Socket.h
index 74084044db..a38d1bb60d 100644
--- a/Source/Core/Core/IPC_HLE/WII_Socket.h
+++ b/Source/Core/Core/IPC_HLE/WII_Socket.h
@@ -199,7 +199,7 @@ public:
 class WiiSockMan : public ::NonCopyable
 {
 public:
-	static s32 GetNetErrorCode(s32 ret, std::string caller, bool isRW);
+	static s32 GetNetErrorCode(s32 ret, const std::string& caller, bool isRW);
 	static char* DecodeError(s32 ErrorCode);
 
 	static WiiSockMan& GetInstance()
diff --git a/Source/Core/Core/NetPlayClient.cpp b/Source/Core/Core/NetPlayClient.cpp
index a32b90f477..3177dd193e 100644
--- a/Source/Core/Core/NetPlayClient.cpp
+++ b/Source/Core/Core/NetPlayClient.cpp
@@ -57,7 +57,7 @@ NetPlayClient::~NetPlayClient()
 }
 
 // called from ---GUI--- thread
-NetPlayClient::NetPlayClient(const std::string& address, const u16 port, NetPlayUI* dialog, const std::string& name, bool traversal, std::string centralServer, u16 centralPort)
+NetPlayClient::NetPlayClient(const std::string& address, const u16 port, NetPlayUI* dialog, const std::string& name, bool traversal, const std::string& centralServer, u16 centralPort)
 	: m_state(Failure)
 	, m_dialog(dialog)
 	, m_client(nullptr)
diff --git a/Source/Core/Core/NetPlayClient.h b/Source/Core/Core/NetPlayClient.h
index 1aa3d2c221..7d3bbf7d73 100644
--- a/Source/Core/Core/NetPlayClient.h
+++ b/Source/Core/Core/NetPlayClient.h
@@ -49,7 +49,7 @@ public:
 	void ThreadFunc();
 	void SendAsync(sf::Packet* packet);
 
-	NetPlayClient(const std::string& address, const u16 port, NetPlayUI* dialog, const std::string& name, bool traversal, std::string centralServer, u16 centralPort);
+	NetPlayClient(const std::string& address, const u16 port, NetPlayUI* dialog, const std::string& name, bool traversal, const std::string& centralServer, u16 centralPort);
 	~NetPlayClient();
 
 	void GetPlayerList(std::string& list, std::vector<int>& pid_list);
diff --git a/Source/Core/Core/NetPlayServer.cpp b/Source/Core/Core/NetPlayServer.cpp
index 7cce306074..f53793f3be 100644
--- a/Source/Core/Core/NetPlayServer.cpp
+++ b/Source/Core/Core/NetPlayServer.cpp
@@ -52,7 +52,7 @@ NetPlayServer::~NetPlayServer()
 }
 
 // called from ---GUI--- thread
-NetPlayServer::NetPlayServer(const u16 port, bool traversal, std::string centralServer, u16 centralPort)
+NetPlayServer::NetPlayServer(const u16 port, bool traversal, const std::string& centralServer, u16 centralPort)
 	: is_connected(false)
 	, m_is_running(false)
 	, m_do_loop(false)
@@ -717,7 +717,7 @@ std::unordered_set<std::string> NetPlayServer::GetInterfaceSet()
 }
 
 // called from ---GUI--- thread
-std::string NetPlayServer::GetInterfaceHost(const std::string inter)
+std::string NetPlayServer::GetInterfaceHost(const std::string& inter)
 {
 	char buf[16];
 	sprintf(buf, ":%d", GetPort());
diff --git a/Source/Core/Core/NetPlayServer.h b/Source/Core/Core/NetPlayServer.h
index e172e012bd..73f8d6d85e 100644
--- a/Source/Core/Core/NetPlayServer.h
+++ b/Source/Core/Core/NetPlayServer.h
@@ -23,7 +23,7 @@ public:
 	void ThreadFunc();
 	void SendAsyncToClients(sf::Packet* packet);
 
-	NetPlayServer(const u16 port, bool traversal, std::string centralServer, u16 centralPort);
+	NetPlayServer(const u16 port, bool traversal, const std::string& centralServer, u16 centralPort);
 	~NetPlayServer();
 
 	bool ChangeGame(const std::string& game);
@@ -47,7 +47,7 @@ public:
 
 	void SetNetPlayUI(NetPlayUI* dialog);
 	std::unordered_set<std::string> GetInterfaceSet();
-	std::string GetInterfaceHost(const std::string inter);
+	std::string GetInterfaceHost(const std::string& inter);
 
 	bool is_connected;
 
diff --git a/Source/Core/DiscIO/NANDContentLoader.cpp b/Source/Core/DiscIO/NANDContentLoader.cpp
index 715ac6e878..8c123da379 100644
--- a/Source/Core/DiscIO/NANDContentLoader.cpp
+++ b/Source/Core/DiscIO/NANDContentLoader.cpp
@@ -439,7 +439,7 @@ void cUIDsys::GetTitleIDs(std::vector<u64>& _TitleIDs, bool _owned)
 	}
 }
 
-u64 CNANDContentManager::Install_WiiWAD(std::string &fileName)
+u64 CNANDContentManager::Install_WiiWAD(const std::string& fileName)
 {
 	if (fileName.find(".wad") == std::string::npos)
 		return 0;
diff --git a/Source/Core/DiscIO/NANDContentLoader.h b/Source/Core/DiscIO/NANDContentLoader.h
index bb380e6dc6..8b309b13f5 100644
--- a/Source/Core/DiscIO/NANDContentLoader.h
+++ b/Source/Core/DiscIO/NANDContentLoader.h
@@ -69,7 +69,7 @@ class CNANDContentManager
 {
 public:
 	static CNANDContentManager& Access() { static CNANDContentManager instance; return instance; }
-	u64 Install_WiiWAD(std::string &fileName);
+	u64 Install_WiiWAD(const std::string& fileName);
 
 	const INANDContentLoader& GetNANDLoader(const std::string& _rName, bool forceReload = false);
 	const INANDContentLoader& GetNANDLoader(u64 _titleId, bool forceReload = false);
diff --git a/Source/Core/InputCommon/ControllerInterface/ExpressionParser.cpp b/Source/Core/InputCommon/ControllerInterface/ExpressionParser.cpp
index abe3043aa1..9837ea74cb 100644
--- a/Source/Core/InputCommon/ControllerInterface/ExpressionParser.cpp
+++ b/Source/Core/InputCommon/ControllerInterface/ExpressionParser.cpp
@@ -92,7 +92,7 @@ public:
 	std::string expr;
 	std::string::iterator it;
 
-	Lexer(std::string expr_) : expr(expr_)
+	Lexer(const std::string& expr_) : expr(expr_)
 	{
 		it = expr.begin();
 	}
@@ -568,7 +568,7 @@ Expression::~Expression()
 	delete node;
 }
 
-static ExpressionParseStatus ParseExpressionInner(std::string str, ControlFinder &finder, Expression **expr_out)
+static ExpressionParseStatus ParseExpressionInner(const std::string& str, ControlFinder &finder, Expression **expr_out)
 {
 	ExpressionParseStatus status;
 	Expression *expr;
@@ -592,7 +592,7 @@ static ExpressionParseStatus ParseExpressionInner(std::string str, ControlFinder
 	return EXPRESSION_PARSE_SUCCESS;
 }
 
-ExpressionParseStatus ParseExpression(std::string str, ControlFinder &finder, Expression **expr_out)
+ExpressionParseStatus ParseExpression(const std::string& str, ControlFinder &finder, Expression **expr_out)
 {
 	// Add compatibility with old simple expressions, which are simple
 	// barewords control names.
diff --git a/Source/Core/InputCommon/ControllerInterface/ExpressionParser.h b/Source/Core/InputCommon/ControllerInterface/ExpressionParser.h
index ca33cc2052..9575ff619d 100644
--- a/Source/Core/InputCommon/ControllerInterface/ExpressionParser.h
+++ b/Source/Core/InputCommon/ControllerInterface/ExpressionParser.h
@@ -63,7 +63,7 @@ enum ExpressionParseStatus
 	EXPRESSION_PARSE_NO_DEVICE,
 };
 
-ExpressionParseStatus ParseExpression(std::string expr, ControlFinder &finder, Expression **expr_out);
+ExpressionParseStatus ParseExpression(const std::string& expr, ControlFinder &finder, Expression **expr_out);
 
 }
 }
diff --git a/Source/Core/VideoBackends/OGL/GLExtensions/GLExtensions.cpp b/Source/Core/VideoBackends/OGL/GLExtensions/GLExtensions.cpp
index b86aef5c7e..6777bec627 100644
--- a/Source/Core/VideoBackends/OGL/GLExtensions/GLExtensions.cpp
+++ b/Source/Core/VideoBackends/OGL/GLExtensions/GLExtensions.cpp
@@ -1908,7 +1908,7 @@ namespace GLExtensions
 			_GLVersion = 330; // Get all the fun things
 	}
 
-	static void* GetFuncAddress(std::string name, void **func)
+	static void* GetFuncAddress(const std::string& name, void **func)
 	{
 		*func = GLInterface->GetFuncAddress(name);
 		if (*func == nullptr)
diff --git a/Source/Core/VideoCommon/PostProcessing.cpp b/Source/Core/VideoCommon/PostProcessing.cpp
index 3841bbfd4f..1ee7e142dd 100644
--- a/Source/Core/VideoCommon/PostProcessing.cpp
+++ b/Source/Core/VideoCommon/PostProcessing.cpp
@@ -293,7 +293,7 @@ void PostProcessingShaderConfiguration::ReloadShader()
 	m_current_shader = "";
 }
 
-void PostProcessingShaderConfiguration::SetOptionf(std::string option, int index, float value)
+void PostProcessingShaderConfiguration::SetOptionf(const std::string& option, int index, float value)
 {
 	auto it = m_options.find(option);
 
@@ -302,7 +302,7 @@ void PostProcessingShaderConfiguration::SetOptionf(std::string option, int index
 	m_any_options_dirty = true;
 }
 
-void PostProcessingShaderConfiguration::SetOptioni(std::string option, int index, s32 value)
+void PostProcessingShaderConfiguration::SetOptioni(const std::string& option, int index, s32 value)
 {
 	auto it = m_options.find(option);
 
@@ -311,7 +311,7 @@ void PostProcessingShaderConfiguration::SetOptioni(std::string option, int index
 	m_any_options_dirty = true;
 }
 
-void PostProcessingShaderConfiguration::SetOptionb(std::string option, bool value)
+void PostProcessingShaderConfiguration::SetOptionb(const std::string& option, bool value)
 {
 	auto it = m_options.find(option);
 
diff --git a/Source/Core/VideoCommon/PostProcessing.h b/Source/Core/VideoCommon/PostProcessing.h
index 237337f2fa..52ebf1b71b 100644
--- a/Source/Core/VideoCommon/PostProcessing.h
+++ b/Source/Core/VideoCommon/PostProcessing.h
@@ -68,9 +68,9 @@ public:
 	const ConfigurationOption& GetOption(const std::string& option) { return m_options[option]; }
 
 	// For updating option's values
-	void SetOptionf(std::string option, int index, float value);
-	void SetOptioni(std::string option, int index, s32 value);
-	void SetOptionb(std::string option, bool value);
+	void SetOptionf(const std::string& option, int index, float value);
+	void SetOptioni(const std::string& option, int index, s32 value);
+	void SetOptionb(const std::string& option, bool value);
 
 private:
 	bool m_any_options_dirty;