Hi there.
Are you sure the issue is due to the kernel upgrade? That is, could you replicate it on F39: performance issues with kernel 6.8+ but none with kernel 6.7?
Is there a reason for not using the Nvidia GPU? The 470.xx driver matching that legacy GPU is still available from RPM Fusion.
There were some changes in GTK graphics renderers introduced with GNOME46 (shipped with F40 and GNOME) and then again on GNOME47 (shipped with F41), but those were rather affecting GTK4 apps. See this for a possible fix.
Could you provide the output from inxi -Fzxx?