diff --git a/include/scratchcpp/block.h b/include/scratchcpp/block.h
index f632420a..641bd1fb 100644
--- a/include/scratchcpp/block.h
+++ b/include/scratchcpp/block.h
@@ -39,13 +39,13 @@ class LIBSCRATCHCPP_EXPORT Block : public Entity
         void setParent(std::shared_ptr<Block> block);
         void setParentId(const std::string &id);
 
-        std::vector<std::shared_ptr<Input>> inputs() const;
+        const std::vector<std::shared_ptr<Input>> &inputs() const;
         int addInput(std::shared_ptr<Input> input);
         std::shared_ptr<Input> inputAt(int index) const;
         int findInput(const std::string &inputName) const;
         Input *findInputById(int id) const;
 
-        std::vector<std::shared_ptr<Field>> fields() const;
+        const std::vector<std::shared_ptr<Field>> &fields() const;
         int addField(std::shared_ptr<Field> field);
         std::shared_ptr<Field> fieldAt(int index) const;
         int findField(const std::string &fieldName) const;
diff --git a/src/engine/internal/engine.cpp b/src/engine/internal/engine.cpp
index 1487a536..3683761d 100644
--- a/src/engine/internal/engine.cpp
+++ b/src/engine/internal/engine.cpp
@@ -56,7 +56,7 @@ void Engine::resolveIds()
 {
     for (auto target : m_targets) {
         std::cout << "Processing target " << target->name() << "..." << std::endl;
-        auto blocks = target->blocks();
+        const auto &blocks = target->blocks();
         for (auto block : blocks) {
             auto container = blockSectionContainer(block->opcode());
             block->setNext(getBlock(block->nextId()));
@@ -64,8 +64,8 @@ void Engine::resolveIds()
             if (container)
                 block->setCompileFunction(container->resolveBlockCompileFunc(block->opcode()));
 
-            auto inputs = block->inputs();
-            for (auto input : inputs) {
+            const auto &inputs = block->inputs();
+            for (const auto &input : inputs) {
                 input->setValueBlock(getBlock(input->valueBlockId()));
                 if (container)
                     input->setInputId(container->resolveInput(input->name()));
@@ -73,7 +73,7 @@ void Engine::resolveIds()
                 input->secondaryValue()->setValuePtr(getEntity(input->primaryValue()->valueId()));
             }
 
-            auto fields = block->fields();
+            const auto &fields = block->fields();
             for (auto field : fields) {
                 field->setValuePtr(getEntity(field->valueId()));
                 if (container) {
@@ -107,7 +107,7 @@ void Engine::compile()
         std::cout << "Compiling scripts in target " << target->name() << "..." << std::endl;
         std::unordered_map<std::string, unsigned int *> procedureBytecodeMap;
         Compiler compiler(this, target.get());
-        auto blocks = target->blocks();
+        const auto &blocks = target->blocks();
         for (auto block : blocks) {
             if (block->topLevel() && !block->shadow()) {
                 auto section = blockSection(block->opcode());
diff --git a/src/internal/iprojectreader.h b/src/internal/iprojectreader.h
index 2e85cfe2..9bda4ed4 100644
--- a/src/internal/iprojectreader.h
+++ b/src/internal/iprojectreader.h
@@ -23,16 +23,16 @@ class IProjectReader
     public:
         virtual ~IProjectReader() { }
 
-        virtual std::string fileName() const final { return m_fileName; }
+        virtual const std::string &fileName() const final { return m_fileName; }
         virtual void setFileName(const std::string &fileName) final { m_fileName = fileName; }
 
         virtual bool load() = 0;
         virtual bool loadData(const std::string &data) = 0;
         virtual bool isValid() = 0;
         virtual void clear() = 0;
-        virtual std::vector<std::shared_ptr<Target>> targets() = 0;
-        virtual std::vector<std::shared_ptr<Broadcast>> broadcasts() = 0;
-        virtual std::vector<std::string> extensions() = 0;
+        virtual const std::vector<std::shared_ptr<Target>> &targets() = 0;
+        virtual const std::vector<std::shared_ptr<Broadcast>> &broadcasts() = 0;
+        virtual const std::vector<std::string> &extensions() = 0;
 
     protected:
         virtual void printErr(const std::string &errStr) final { std::cerr << "Failed to read project: " << errStr << std::endl; }
diff --git a/src/internal/scratch3reader.cpp b/src/internal/scratch3reader.cpp
index 6a5e2736..14b4e454 100644
--- a/src/internal/scratch3reader.cpp
+++ b/src/internal/scratch3reader.cpp
@@ -388,17 +388,17 @@ void Scratch3Reader::clear()
     m_extensions.clear();
 }
 
-std::vector<std::shared_ptr<Target>> Scratch3Reader::targets()
+const std::vector<std::shared_ptr<Target>> &Scratch3Reader::targets()
 {
     return m_targets;
 }
 
-std::vector<std::shared_ptr<Broadcast>> Scratch3Reader::broadcasts()
+const std::vector<std::shared_ptr<Broadcast>> &Scratch3Reader::broadcasts()
 {
     return m_broadcasts;
 }
 
-std::vector<std::string> Scratch3Reader::extensions()
+const std::vector<std::string> &Scratch3Reader::extensions()
 {
     return m_extensions;
 }
diff --git a/src/internal/scratch3reader.h b/src/internal/scratch3reader.h
index cd6d78dd..88fc6c20 100644
--- a/src/internal/scratch3reader.h
+++ b/src/internal/scratch3reader.h
@@ -17,9 +17,9 @@ class Scratch3Reader : public IProjectReader
         bool loadData(const std::string &data) override;
         bool isValid() override;
         void clear() override;
-        std::vector<std::shared_ptr<Target>> targets() override;
-        std::vector<std::shared_ptr<Broadcast>> broadcasts() override;
-        std::vector<std::string> extensions() override;
+        const std::vector<std::shared_ptr<Target>> &targets() override;
+        const std::vector<std::shared_ptr<Broadcast>> &broadcasts() override;
+        const std::vector<std::string> &extensions() override;
 
     private:
         void read();
diff --git a/src/scratch/block.cpp b/src/scratch/block.cpp
index b36e5f48..0dea5897 100644
--- a/src/scratch/block.cpp
+++ b/src/scratch/block.cpp
@@ -155,7 +155,7 @@ void Block::setParentId(const std::string &id)
 }
 
 /*! Returns the list of inputs. */
-std::vector<std::shared_ptr<Input>> Block::inputs() const
+const std::vector<std::shared_ptr<Input>> &Block::inputs() const
 {
     return impl->inputs;
 }
@@ -217,7 +217,7 @@ void Block::updateInputMap()
 }
 
 /*! Returns the list of fields. */
-std::vector<std::shared_ptr<Field>> Block::fields() const
+const std::vector<std::shared_ptr<Field>> &Block::fields() const
 {
     return impl->fields;
 }