This observation came from https://bugreports.qt.io/browse/QTBUG-92904 ... Qt syncs the GUI and render thread every frame when an Animator is running, although there really isn't anything to sync at all while the animator is running.

I have created 2 WIP patches against qtbase and qtdeclarative (dev/6.2) that "fix" this behavior and free up the GUI thread completely while Animators are running:

https://codereview.qt-project.org/c/qt/qtbase/+/350313
https://codereview.qt-project.org/c/qt/qtdeclarative/+/350314

Currently, only the normal threaded renderloop is patched.

I've also attached a test program to verify that the blockedForSync intervals are completely gone for Animators after the patch is applied: no-render-sync.tgz

Currently tested on Linux against both dev and 5.15 (the qtdeclarative patch doesn't apply cleanly there and needs manual adjustments)