That's normal and a feature. Mouse pointer is driven by hardware input and rendered independently (was a hardware overlay in the old days) and is soft realtime (hopefully). Because the window manager is a normal process, operating asynchronoysly, the window frame will lag a bit (depends on system load), and the app contents even more (this can't be avoided, even if the WM/compositor was realtime). Making the window frame wait for window content is broken by design.