[Reg 6.5.2 -> 6.8.1] Significant increase in CPU load in QSGRenderThread

See the attached sample project (it's not very minimal right now; I'll upload a stripped-down reproducer when I have time).

• With Boot to Qt 6.5.2, my Raspberry Pi 4 runs at ~60% CPU load
• With Boot to Qt 6.8.1/6.8.3, my Raspberry Pi 4 runs at ~80% CPU load

Other observations

• When using perf record -e cycles -p $(pidof QQuickTableViewSampleApp) sleep 30 Qt 6.8 seems to record ~20k more CPU events than Qt 6.5.2 in the same 30-second window.
• The CPU load is negligible on my Windows 10 22H2 PC (i7-770 CPU)