diff --git a/src/engine/idtreedb.cpp b/src/engine/idtreedb.cpp
--- a/src/engine/idtreedb.cpp
+++ b/src/engine/idtreedb.cpp
@@ -116,7 +116,7 @@
 //
 class IdTreePostingIterator : public PostingIterator {
 public:
-    IdTreePostingIterator(const IdTreeDB& db, const QVector<quint64> list)
+    IdTreePostingIterator(const IdTreeDB& db, const QVector<quint64> &list)
         : m_db(db), m_pos(-1), m_idList(list) {}
 
     quint64 docId() const override {
diff --git a/src/engine/writetransaction.h b/src/engine/writetransaction.h
--- a/src/engine/writetransaction.h
+++ b/src/engine/writetransaction.h
@@ -57,7 +57,7 @@
      * This function should typically be called when there are no other ReadTransaction in process
      * as that would otherwise balloon the size of the database.
      */
-    bool removeRecursively(quint64 parentId, std::function<bool(quint64)> shouldDelete);
+    bool removeRecursively(quint64 parentId, const std::function<bool(quint64)> &shouldDelete);
 
     void replaceDocument(const Document& doc, DocumentOperations operations);
     void commit();
diff --git a/src/engine/writetransaction.cpp b/src/engine/writetransaction.cpp
--- a/src/engine/writetransaction.cpp
+++ b/src/engine/writetransaction.cpp
@@ -167,7 +167,7 @@
     removeDocument(parentId);
 }
 
-bool WriteTransaction::removeRecursively(quint64 parentId, std::function<bool(quint64)> shouldDelete)
+bool WriteTransaction::removeRecursively(quint64 parentId, const std::function<bool(quint64)> &shouldDelete)
 {
     DocumentUrlDB docUrlDB(m_dbis.idTreeDbi, m_dbis.idFilenameDbi, m_txn);
 
diff --git a/src/kioslaves/search/kio_search.h b/src/kioslaves/search/kio_search.h
--- a/src/kioslaves/search/kio_search.h
+++ b/src/kioslaves/search/kio_search.h
@@ -54,8 +54,8 @@
     void stat(const QUrl& url) override;
 
 private:
-    QString getUserName(KUserId uid) const;
-    QString getGroupName(KGroupId gid) const;
+    QString getUserName(const KUserId &uid) const;
+    QString getGroupName(const KGroupId &gid) const;
 
     mutable QHash<KUserId, QString> mUsercache;
     mutable QHash<KGroupId, QString> mGroupcache;
diff --git a/src/kioslaves/search/kio_search.cpp b/src/kioslaves/search/kio_search.cpp
--- a/src/kioslaves/search/kio_search.cpp
+++ b/src/kioslaves/search/kio_search.cpp
@@ -181,7 +181,7 @@
     finished();
 }
 
-QString SearchProtocol::getUserName(KUserId uid) const
+QString SearchProtocol::getUserName(const KUserId &uid) const
 {
     if (Q_UNLIKELY(!uid.isValid())) {
         return QString();
@@ -198,7 +198,7 @@
     return mUsercache[uid];
 }
 
-QString SearchProtocol::getGroupName(KGroupId gid) const
+QString SearchProtocol::getGroupName(const KGroupId &gid) const
 {
     if (Q_UNLIKELY(!gid.isValid())) {
         return QString();