diff --git a/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp b/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp
index a2ee8aa84..88f88b30a 100644
--- a/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp
+++ b/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp
@@ -232,7 +232,7 @@ namespace Opm {
                                  const std::vector<double>& zcorn ,
                                  const int * actnum);
 
-        bool keywInputBeforeGdfile(const Deck& deck, const std::string keyword) const;
+        bool keywInputBeforeGdfile(const Deck& deck, const std::string& keyword) const;
 
         void initCylindricalGrid(const Deck&);
         void initSpiderwebGrid(const Deck&);
diff --git a/opm/utility/EModel.hpp b/opm/utility/EModel.hpp
index fcd441f09..3982435a3 100644
--- a/opm/utility/EModel.hpp
+++ b/opm/utility/EModel.hpp
@@ -55,10 +55,10 @@ public:
     void resetFilter();
 
     template <typename T>
-    void addFilter(std::string param1, std::string opperator, T num);
+    void addFilter(const std::string& param1, const std::string& opperator, T num);
 
     template <typename T>
-    void addFilter(std::string param1, std::string opperator, T num1, T num2);
+    void addFilter(const std::string& param1, const std::string& opperator, T num1, T num2);
 
     void setDepthfwl(const std::vector<float>& fwl);
 
@@ -116,13 +116,13 @@ private:
     const std::vector<float>& getSolutionFloat(const std::string& name);
 
     template <typename T>
-    const std::vector<T>& get_filter_param(std::string param1);
+    const std::vector<T>& get_filter_param(const std::string& param1);
 
     template <typename T>
-    void updateActiveFilter(const std::vector<T>& paramVect, const std::string opperator, T value);
+    void updateActiveFilter(const std::vector<T>& paramVect, const std::string& opperator, T value);
 
     template <typename T>
-    void updateActiveFilter(const std::vector<T>& paramVect, const std::string opperator, T value1, T value2);
+    void updateActiveFilter(const std::vector<T>& paramVect, const std::string& opperator, T value1, T value2);
 
 };
 
diff --git a/src/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp b/src/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp
index 12b13ef02..b7cab2b8b 100644
--- a/src/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp
+++ b/src/opm/parser/eclipse/EclipseState/Grid/EclipseGrid.cpp
@@ -435,7 +435,7 @@ EclipseGrid::EclipseGrid(const Deck& deck, const int * actnum)
         zcorn_fixed = mapper.fixupZCORN( m_zcorn );
     }
 
-    bool EclipseGrid::keywInputBeforeGdfile(const Deck& deck, const std::string keyword) const {
+    bool EclipseGrid::keywInputBeforeGdfile(const Deck& deck, const std::string& keyword) const {
 
         std::vector<std::string> keywordList;
         keywordList.reserve(deck.size());
diff --git a/src/opm/utility/EModel.cpp b/src/opm/utility/EModel.cpp
index 5902cf6b7..8f487b319 100644
--- a/src/opm/utility/EModel.cpp
+++ b/src/opm/utility/EModel.cpp
@@ -261,7 +261,7 @@ void EModel::resetFilter()
 
 
 template <typename T>
-void EModel::updateActiveFilter(const std::vector<T>& paramVect, const std::string opperator, T value)
+void EModel::updateActiveFilter(const std::vector<T>& paramVect, const std::string& opperator, T value)
 {
     if ((opperator == "eq") || (opperator == "==")){
        for (size_t i = 0; i < paramVect.size(); i++)
@@ -287,7 +287,7 @@ void EModel::updateActiveFilter(const std::vector<T>& paramVect, const std::stri
 }
 
 template <typename T>
-void EModel::updateActiveFilter(const std::vector<T>& paramVect, const std::string opperator, T value1, T value2)
+void EModel::updateActiveFilter(const std::vector<T>& paramVect, const std::string& opperator, T value1, T value2)
 {
     if ((opperator == "in") || (opperator == "between")) {
         for (size_t i = 0; i < paramVect.size(); i++)
@@ -302,7 +302,7 @@ void EModel::updateActiveFilter(const std::vector<T>& paramVect, const std::stri
 }
 
 template <typename T>
-const std::vector<T>& EModel::get_filter_param(std::string param)
+const std::vector<T>& EModel::get_filter_param(const std::string& param)
 {
     if constexpr (std::is_same<T, int>::value){
         if ((param == "I") || (param == "ROW"))
@@ -335,21 +335,21 @@ const std::vector<T>& EModel::get_filter_param(std::string param)
 
 
 template <>
-void EModel::addFilter<int>(std::string param1, std::string opperator, int num)
+void EModel::addFilter<int>(const std::string& param1, const std::string& opperator, int num)
 {
     std::vector<int> paramVect = get_filter_param<int>(param1);
     updateActiveFilter(paramVect, opperator, num);
 }
 
 template <>
-void EModel::addFilter<int>(std::string param1, std::string opperator, int num1, int num2)
+void EModel::addFilter<int>(const std::string& param1, const std::string& opperator, int num1, int num2)
 {
     std::vector<int> paramVect = get_filter_param<int>(param1);;
     updateActiveFilter(paramVect, opperator, num1, num2);
 }
 
 template <>
-void EModel::addFilter<float>(std::string param1, std::string opperator, float num)
+void EModel::addFilter<float>(const std::string& param1, const std::string& opperator, float num)
 {
     std::vector<float> paramVect = get_filter_param<float>(param1);
     updateActiveFilter(paramVect, opperator, num);
@@ -357,7 +357,7 @@ void EModel::addFilter<float>(std::string param1, std::string opperator, float n
 
 
 template <>
-void EModel::addFilter<float>(std::string param1, std::string opperator, float num1, float num2)
+void EModel::addFilter<float>(const std::string& param1, const std::string& opperator, float num1, float num2)
 {
     std::vector<float> paramVect = get_filter_param<float>(param1);
     updateActiveFilter(paramVect, opperator, num1, num2);