diff --git a/xs/src/libslic3r/Config.cpp b/xs/src/libslic3r/Config.cpp
index 741151c38..e388d8f9b 100644
--- a/xs/src/libslic3r/Config.cpp
+++ b/xs/src/libslic3r/Config.cpp
@@ -8,7 +8,7 @@
 namespace Slic3r {
 
 bool
-ConfigBase::has(const t_config_option_key opt_key) {
+ConfigBase::has(const t_config_option_key &opt_key) {
     return (this->option(opt_key, false) != NULL);
 }
 
@@ -52,14 +52,14 @@ ConfigBase::diff(ConfigBase &other) {
 }
 
 std::string
-ConfigBase::serialize(const t_config_option_key opt_key) {
+ConfigBase::serialize(const t_config_option_key &opt_key) {
     ConfigOption* opt = this->option(opt_key);
     assert(opt != NULL);
     return opt->serialize();
 }
 
 bool
-ConfigBase::set_deserialize(const t_config_option_key opt_key, std::string str) {
+ConfigBase::set_deserialize(const t_config_option_key &opt_key, std::string str) {
     if (this->def->count(opt_key) == 0) throw "Calling set_deserialize() on unknown option";
     ConfigOptionDef* optdef = &(*this->def)[opt_key];
     if (!optdef->shortcut.empty()) {
@@ -75,7 +75,7 @@ ConfigBase::set_deserialize(const t_config_option_key opt_key, std::string str)
 }
 
 double
-ConfigBase::get_abs_value(const t_config_option_key opt_key) {
+ConfigBase::get_abs_value(const t_config_option_key &opt_key) {
     ConfigOption* opt = this->option(opt_key, false);
     if (ConfigOptionFloatOrPercent* optv = dynamic_cast<ConfigOptionFloatOrPercent*>(opt)) {
         // get option definition
@@ -92,7 +92,7 @@ ConfigBase::get_abs_value(const t_config_option_key opt_key) {
 }
 
 double
-ConfigBase::get_abs_value(const t_config_option_key opt_key, double ratio_over) {
+ConfigBase::get_abs_value(const t_config_option_key &opt_key, double ratio_over) {
     // get stored option value
     ConfigOptionFloatOrPercent* opt = dynamic_cast<ConfigOptionFloatOrPercent*>(this->option(opt_key));
     assert(opt != NULL);
@@ -282,7 +282,7 @@ ConfigBase::set(t_config_option_key opt_key, SV* value) {
 /* This method is implemented as a workaround for this typemap bug:
    https://rt.cpan.org/Public/Bug/Display.html?id=94110 */
 bool
-ConfigBase::set_deserialize(const t_config_option_key opt_key, SV* str) {
+ConfigBase::set_deserialize(const t_config_option_key &opt_key, SV* str) {
     size_t len;
     const char * c = SvPV(str, len);
     std::string value(c, len);
@@ -328,7 +328,7 @@ DynamicConfig::DynamicConfig (const DynamicConfig& other) {
 }
 
 ConfigOption*
-DynamicConfig::option(const t_config_option_key opt_key, bool create) {
+DynamicConfig::option(const t_config_option_key &opt_key, bool create) {
     if (this->options.count(opt_key) == 0) {
         if (create) {
             ConfigOptionDef* optdef = &(*this->def)[opt_key];
@@ -375,16 +375,16 @@ DynamicConfig::option(const t_config_option_key opt_key, bool create) {
 
 template<class T>
 T*
-DynamicConfig::opt(const t_config_option_key opt_key, bool create) {
+DynamicConfig::opt(const t_config_option_key &opt_key, bool create) {
     return dynamic_cast<T*>(this->option(opt_key, create));
 }
-template ConfigOptionInt* DynamicConfig::opt<ConfigOptionInt>(const t_config_option_key opt_key, bool create);
-template ConfigOptionBool* DynamicConfig::opt<ConfigOptionBool>(const t_config_option_key opt_key, bool create);
-template ConfigOptionBools* DynamicConfig::opt<ConfigOptionBools>(const t_config_option_key opt_key, bool create);
-template ConfigOptionPercent* DynamicConfig::opt<ConfigOptionPercent>(const t_config_option_key opt_key, bool create);
+template ConfigOptionInt* DynamicConfig::opt<ConfigOptionInt>(const t_config_option_key &opt_key, bool create);
+template ConfigOptionBool* DynamicConfig::opt<ConfigOptionBool>(const t_config_option_key &opt_key, bool create);
+template ConfigOptionBools* DynamicConfig::opt<ConfigOptionBools>(const t_config_option_key &opt_key, bool create);
+template ConfigOptionPercent* DynamicConfig::opt<ConfigOptionPercent>(const t_config_option_key &opt_key, bool create);
 
 const ConfigOption*
-DynamicConfig::option(const t_config_option_key opt_key) const {
+DynamicConfig::option(const t_config_option_key &opt_key) const {
     return const_cast<DynamicConfig*>(this)->option(opt_key, false);
 }
 
@@ -397,7 +397,7 @@ DynamicConfig::keys() const {
 }
 
 void
-DynamicConfig::erase(const t_config_option_key opt_key) {
+DynamicConfig::erase(const t_config_option_key &opt_key) {
     this->options.erase(opt_key);
 }
 
@@ -412,7 +412,7 @@ StaticConfig::keys() const {
 }
 
 const ConfigOption*
-StaticConfig::option(const t_config_option_key opt_key) const
+StaticConfig::option(const t_config_option_key &opt_key) const
 {
     return const_cast<StaticConfig*>(this)->option(opt_key, false);
 }
diff --git a/xs/src/libslic3r/Config.hpp b/xs/src/libslic3r/Config.hpp
index be433633d..2d192d235 100644
--- a/xs/src/libslic3r/Config.hpp
+++ b/xs/src/libslic3r/Config.hpp
@@ -512,18 +512,18 @@ class ConfigBase
     t_optiondef_map* def;
     
     ConfigBase() : def(NULL) {};
-    bool has(const t_config_option_key opt_key);
-    virtual ConfigOption* option(const t_config_option_key opt_key, bool create = false) = 0;
-    virtual const ConfigOption* option(const t_config_option_key opt_key) const = 0;
+    bool has(const t_config_option_key &opt_key);
+    virtual ConfigOption* option(const t_config_option_key &opt_key, bool create = false) = 0;
+    virtual const ConfigOption* option(const t_config_option_key &opt_key) const = 0;
     virtual t_config_option_keys keys() const = 0;
     void apply(const ConfigBase &other, bool ignore_nonexistent = false);
     bool equals(ConfigBase &other);
     t_config_option_keys diff(ConfigBase &other);
-    std::string serialize(const t_config_option_key opt_key);
-    bool set_deserialize(const t_config_option_key opt_key, std::string str);
+    std::string serialize(const t_config_option_key &opt_key);
+    bool set_deserialize(const t_config_option_key &opt_key, std::string str);
     void set_ifndef(t_config_option_key opt_key, SV* value, bool deserialize = false);
-    double get_abs_value(const t_config_option_key opt_key);
-    double get_abs_value(const t_config_option_key opt_key, double ratio_over);
+    double get_abs_value(const t_config_option_key &opt_key);
+    double get_abs_value(const t_config_option_key &opt_key, double ratio_over);
     void setenv_();
     
     #ifdef SLIC3RXS
@@ -531,7 +531,7 @@ class ConfigBase
     SV* get(t_config_option_key opt_key);
     SV* get_at(t_config_option_key opt_key, size_t i);
     bool set(t_config_option_key opt_key, SV* value);
-    bool set_deserialize(const t_config_option_key opt_key, SV* str);
+    bool set_deserialize(const t_config_option_key &opt_key, SV* str);
     #endif
 };
 
@@ -543,11 +543,11 @@ class DynamicConfig : public ConfigBase
     DynamicConfig& operator= (DynamicConfig other);
     void swap(DynamicConfig &other);
     ~DynamicConfig();
-    template<class T> T* opt(const t_config_option_key opt_key, bool create = false);
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false);
-    const ConfigOption* option(const t_config_option_key opt_key) const;
+    template<class T> T* opt(const t_config_option_key &opt_key, bool create = false);
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false);
+    const ConfigOption* option(const t_config_option_key &opt_key) const;
     t_config_option_keys keys() const;
-    void erase(const t_config_option_key opt_key);
+    void erase(const t_config_option_key &opt_key);
     
     private:
     typedef std::map<t_config_option_key,ConfigOption*> t_options_map;
@@ -558,8 +558,8 @@ class StaticConfig : public ConfigBase
 {
     public:
     t_config_option_keys keys() const;
-    virtual ConfigOption* option(const t_config_option_key opt_key, bool create = false) = 0;
-    const ConfigOption* option(const t_config_option_key opt_key) const;
+    virtual ConfigOption* option(const t_config_option_key &opt_key, bool create = false) = 0;
+    const ConfigOption* option(const t_config_option_key &opt_key) const;
     
     #ifdef SLIC3RXS
     bool set(t_config_option_key opt_key, SV* value);
diff --git a/xs/src/libslic3r/PrintConfig.hpp b/xs/src/libslic3r/PrintConfig.hpp
index 012135e01..5e8d542c9 100644
--- a/xs/src/libslic3r/PrintConfig.hpp
+++ b/xs/src/libslic3r/PrintConfig.hpp
@@ -150,7 +150,7 @@ class PrintObjectConfig : public virtual StaticPrintConfig
         this->xy_size_compensation.value                         = 0;
     };
     
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false) {
         OPT_PTR(dont_support_bridges);
         OPT_PTR(extrusion_width);
         OPT_PTR(first_layer_height);
@@ -260,7 +260,7 @@ class PrintRegionConfig : public virtual StaticPrintConfig
         this->top_solid_layers.value                             = 3;
     };
     
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false) {
         OPT_PTR(bottom_solid_layers);
         OPT_PTR(bridge_flow_ratio);
         OPT_PTR(bridge_speed);
@@ -359,7 +359,7 @@ class GCodeConfig : public virtual StaticPrintConfig
         this->use_volumetric_e.value                             = false;
     };
     
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false) {
         OPT_PTR(before_layer_gcode);
         OPT_PTR(end_gcode);
         OPT_PTR(extrusion_axis);
@@ -518,7 +518,7 @@ class PrintConfig : public GCodeConfig
         this->z_offset.value                                     = 0;
     };
     
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false) {
         OPT_PTR(avoid_crossing_perimeters);
         OPT_PTR(bed_shape);
         OPT_PTR(bed_temperature);
@@ -589,7 +589,7 @@ class HostConfig : public virtual StaticPrintConfig
         this->octoprint_apikey.value                            = "";
     };
     
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false) {
         OPT_PTR(octoprint_host);
         OPT_PTR(octoprint_apikey);
         
@@ -601,7 +601,7 @@ class FullPrintConfig
     : public PrintObjectConfig, public PrintRegionConfig, public PrintConfig, public HostConfig
 {
     public:
-    ConfigOption* option(const t_config_option_key opt_key, bool create = false) {
+    ConfigOption* option(const t_config_option_key &opt_key, bool create = false) {
         ConfigOption* opt;
         if ((opt = PrintObjectConfig::option(opt_key, create)) != NULL) return opt;
         if ((opt = PrintRegionConfig::option(opt_key, create)) != NULL) return opt;