diff --git a/src/corelib/thread/qthreadpool.cpp b/src/corelib/thread/qthreadpool.cpp
index c7531111da7..f4dc940cc4b 100644
--- a/src/corelib/thread/qthreadpool.cpp
+++ b/src/corelib/thread/qthreadpool.cpp
@@ -481,8 +481,13 @@ QThreadPool *QThreadPoolPrivate::qtGuiInstance()
     Q_CONSTINIT static QBasicMutex theMutex;
 
     const QMutexLocker locker(&theMutex);
-    if (guiInstance.isNull() && !QCoreApplication::closingDown())
+    if (guiInstance.isNull() && !QCoreApplication::closingDown()) {
         guiInstance = new QThreadPool();
+        // Limit max thread to avoid too many parallel threads.
+        // We are not optimized for much more than 4 or 8 threads.
+        if (guiInstance && guiInstance->maxThreadCount() > 4)
+            guiInstance->setMaxThreadCount(qBound(4, guiInstance->maxThreadCount() / 2, 8));
+    }
     return guiInstance;
 }