diff --git a/include/Screeps/Creep.hpp b/include/Screeps/Creep.hpp
index 89ec4a7..7b2484f 100644
--- a/include/Screeps/Creep.hpp
+++ b/include/Screeps/Creep.hpp
@@ -66,7 +66,7 @@ public:
 
 	int dismantle(const Structure& target);
 
-	int drop(std::string resourceType, std::optional<int> amount);
+	int drop(const std::string& resourceType, std::optional<int> amount);
 
 	int generateSafeMode(const StructureController& target);
 
@@ -109,17 +109,17 @@ public:
 	int suicide();
 
 	int transfer(const Creep& target,
-	             std::string resourceType,
-	             std::optional<int> amount = std::nullopt);
+	             const std::string& resourceType,
+	             const std::optional<int>& amount = std::nullopt);
 	int transfer(const Structure& target,
-	             std::string resourceType,
-	             std::optional<int> amount = std::nullopt);
+	             const std::string& resourceType,
+	             const std::optional<int>& amount = std::nullopt);
 
 	int upgradeController(const StructureController& target);
 
 	int withdraw(const RoomObject& target,
-	             std::string resourceType,
-	             std::optional<int> amount = std::nullopt);
+	             const std::string& resourceType,
+	             const std::optional<int>& amount = std::nullopt);
 };
 
 } // namespace Screeps
diff --git a/include/Screeps/Flag.hpp b/include/Screeps/Flag.hpp
index 9dfdd19..f3e563e 100644
--- a/include/Screeps/Flag.hpp
+++ b/include/Screeps/Flag.hpp
@@ -21,7 +21,7 @@ public:
 
 	void remove();
 
-	int setColor(int color, std::optional<int> secondaryColor);
+	int setColor(int color, const std::optional<int>& secondaryColor);
 
 	int setPosition(int x, int y);
 	int setPosition(const RoomPosition& pos);
diff --git a/include/Screeps/Game.hpp b/include/Screeps/Game.hpp
index 17b4189..d0ced0f 100644
--- a/include/Screeps/Game.hpp
+++ b/include/Screeps/Game.hpp
@@ -74,7 +74,7 @@ public:
 
 	void cpuHalt();
 
-	int cpuSetShardLimits(std::map<std::string, int> limits);
+	int cpuSetShardLimits(const std::map<std::string, int>& limits);
 
 	int cpuUnlock();
 
diff --git a/include/Screeps/JS.hpp b/include/Screeps/JS.hpp
index ce79c24..e0ebc0c 100644
--- a/include/Screeps/JS.hpp
+++ b/include/Screeps/JS.hpp
@@ -21,7 +21,7 @@ const static Value gGlobal = Value::global();
 const static Value gObject = Value::global("Object");
 
 Value getGlobal(char const* name);
-Value getGlobal(std::string const& name);
+Value getGlobal(const std::string& name);
 Value getConstant(const std::string& name);
 
 bool isInstanceOf(const Value& val, const char* name);
diff --git a/include/Screeps/Room.hpp b/include/Screeps/Room.hpp
index cbf7c5f..d101fec 100644
--- a/include/Screeps/Room.hpp
+++ b/include/Screeps/Room.hpp
@@ -35,25 +35,25 @@ public:
 
 	// std::string serializePath(...)
 
-	// ... deserializePath(const std::string path);
+	// ... deserializePath(const std::string& path);
 
 	int createConstructionSite(int x,
 	                           int y,
-	                           std::string structureType,
-	                           std::optional<std::string> name = std::nullopt);
+	                           const std::string& structureType,
+	                           const std::optional<std::string>& name = std::nullopt);
 	int createConstructionSite(const RoomPosition& pos,
-	                           std::string structureType,
-	                           std::optional<std::string> name = std::nullopt);
+	                           const std::string& structureType,
+	                           const std::optional<std::string>& name = std::nullopt);
 
 	int createFlag(int x,
 	               int y,
-	               std::optional<std::string> name = std::nullopt,
-	               std::optional<std::string> color = std::nullopt,
-	               std::optional<std::string> secondaryColor = std::nullopt);
+	               const std::optional<std::string>& name = std::nullopt,
+	               const std::optional<std::string>& color = std::nullopt,
+	               const std::optional<std::string>& secondaryColor = std::nullopt);
 	int createFlag(const RoomPosition& pos,
-	               std::optional<std::string> name = std::nullopt,
-	               std::optional<std::string> color = std::nullopt,
-	               std::optional<std::string> secondaryColor = std::nullopt);
+	               const std::optional<std::string>& name = std::nullopt,
+	               const std::optional<std::string>& color = std::nullopt,
+	               const std::optional<std::string>& secondaryColor = std::nullopt);
 
 	std::vector<std::unique_ptr<RoomObject>>
 	find(int type, std::function<bool(const JS::Value&)> predicate = {}) const;
@@ -87,7 +87,7 @@ public:
 	// ... lookAt(int x, int y);
 	// ... lookAt(const RoomPosition& target);
 
-	// ... lookAtArea(int top, int left, int bottom, int right, std::optional<bool> asArray = std::nullopt);
+	// ... lookAtArea(int top, int left, int bottom, int right, const std::optional<bool>& asArray = std::nullopt);
 
 	// ... lookForAt(const std::string& type, int x, int y);
 	// ... lookForAt(const std::string& type, const RoomPosition& target);
@@ -97,7 +97,7 @@ public:
 	//                int left,
 	//                int bottom,
 	//                int right,
-	//                std::optional<bool> asArray = std::nullopt);
+	//                const std::optional<bool>& asArray = std::nullopt);
 };
 
 } // namespace Screeps
diff --git a/include/Screeps/StructureLink.hpp b/include/Screeps/StructureLink.hpp
index a11622b..f3e8eee 100644
--- a/include/Screeps/StructureLink.hpp
+++ b/include/Screeps/StructureLink.hpp
@@ -16,7 +16,7 @@ public:
 
 	Store store() const;
 
-	int transferEnergy(const StructureLink& target, std::optional<int> amount = std::nullopt);
+	int transferEnergy(const StructureLink& target, const std::optional<int>& amount = std::nullopt);
 };
 
 } // namespace Screeps
diff --git a/include/Screeps/StructureSpawn.hpp b/include/Screeps/StructureSpawn.hpp
index ef7b734..062b925 100644
--- a/include/Screeps/StructureSpawn.hpp
+++ b/include/Screeps/StructureSpawn.hpp
@@ -51,7 +51,7 @@ public:
 
 	int cancel();
 
-	int setDirections(std::vector<int> directions);
+	int setDirections(const std::vector<int>& directions);
 };
 
 } // namespace Screeps
diff --git a/include/Screeps/StructureTerminal.hpp b/include/Screeps/StructureTerminal.hpp
index 3bc69f7..f5a1a96 100644
--- a/include/Screeps/StructureTerminal.hpp
+++ b/include/Screeps/StructureTerminal.hpp
@@ -20,7 +20,7 @@ public:
 	int send(const std::string& resourceType,
 	         int amount,
 	         const std::string& destination,
-	         std::optional<std::string> description = std::nullopt);
+	         const std::optional<std::string>& description = std::nullopt);
 };
 
 } // namespace Screeps
diff --git a/src/Creep.cpp b/src/Creep.cpp
index d568e56..7ead81b 100644
--- a/src/Creep.cpp
+++ b/src/Creep.cpp
@@ -125,7 +125,7 @@ int Creep::dismantle(const Structure& target)
 	return value().call<int>("dismantle", target.value());
 }
 
-int Creep::drop(std::string resourceType, std::optional<int> amount)
+int Creep::drop(const std::string& resourceType, std::optional<int> amount)
 {
 	if (amount)
 		return value().call<int>("drop", resourceType, *amount);
@@ -247,7 +247,9 @@ int Creep::suicide()
 	return value().call<int>("suicide");
 }
 
-int Creep::transfer(const Creep& target, std::string resourceType, std::optional<int> amount)
+int Creep::transfer(const Creep& target,
+                    const std::string& resourceType,
+                    const std::optional<int>& amount)
 {
 	if (amount)
 		return value().call<int>("transfer", target.value(), resourceType, *amount);
@@ -255,7 +257,9 @@ int Creep::transfer(const Creep& target, std::string resourceType, std::optional
 		return value().call<int>("transfer", target.value(), resourceType);
 }
 
-int Creep::transfer(const Structure& target, std::string resourceType, std::optional<int> amount)
+int Creep::transfer(const Structure& target,
+                    const std::string& resourceType,
+                    const std::optional<int>& amount)
 {
 	if (amount)
 		return value().call<int>("transfer", target.value(), resourceType, *amount);
@@ -268,7 +272,9 @@ int Creep::upgradeController(const StructureController& target)
 	return value().call<int>("upgradeController", target.value());
 }
 
-int Creep::withdraw(const RoomObject& target, std::string resourceType, std::optional<int> amount)
+int Creep::withdraw(const RoomObject& target,
+                    const std::string& resourceType,
+                    const std::optional<int>& amount)
 {
 	if (amount)
 		return value().call<int>("withdraw", target.value(), resourceType, *amount);
diff --git a/src/Flag.cpp b/src/Flag.cpp
index fa03790..231d7a8 100644
--- a/src/Flag.cpp
+++ b/src/Flag.cpp
@@ -39,7 +39,7 @@ void Flag::remove()
 	value().call<void>("remove");
 }
 
-int Flag::setColor(int color, std::optional<int> secondaryColor)
+int Flag::setColor(int color, const std::optional<int>& secondaryColor)
 {
 	if (secondaryColor)
 		return value().call<int>("setColor", color, *secondaryColor);
diff --git a/src/JS.cpp b/src/JS.cpp
index 93b161d..bb3a05d 100644
--- a/src/JS.cpp
+++ b/src/JS.cpp
@@ -9,7 +9,7 @@ Value getGlobal(char const* name)
 	return Value::global(name);
 }
 
-Value getGlobal(std::string const& name)
+Value getGlobal(const std::string& name)
 {
 	return Value::global(name.c_str());
 }
diff --git a/src/StructureLink.cpp b/src/StructureLink.cpp
index 422c018..8089c94 100644
--- a/src/StructureLink.cpp
+++ b/src/StructureLink.cpp
@@ -18,7 +18,7 @@ Store StructureLink::store() const
 	return Store(value()["store"]);
 }
 
-int StructureLink::transferEnergy(const StructureLink& target, std::optional<int> amount)
+int StructureLink::transferEnergy(const StructureLink& target, const std::optional<int>& amount)
 {
 	if (amount)
 		return value().call<int>("transferEnergy", target.value(), *amount);
diff --git a/src/StructureSpawn.cpp b/src/StructureSpawn.cpp
index 0d4a3e0..3009a8f 100644
--- a/src/StructureSpawn.cpp
+++ b/src/StructureSpawn.cpp
@@ -99,7 +99,7 @@ int StructureSpawn::Spawning::cancel()
 	return value().call<int>("cancel");
 }
 
-int StructureSpawn::Spawning::setDirections(std::vector<int> directions)
+int StructureSpawn::Spawning::setDirections(const std::vector<int>& directions)
 {
 	return value().call<int>("setDirections", JS::vectorToJSArray(directions));
 }
diff --git a/src/StructureTerminal.cpp b/src/StructureTerminal.cpp
index 3093f69..2dd067b 100644
--- a/src/StructureTerminal.cpp
+++ b/src/StructureTerminal.cpp
@@ -19,7 +19,7 @@ Store StructureTerminal::store() const
 int StructureTerminal::send(const std::string& resourceType,
                             int amount,
                             const std::string& destination,
-                            std::optional<std::string> description)
+                            const std::optional<std::string>& description)
 {
 	if (description)
 		return value().call<int>("send", resourceType, amount, destination, description);