With latest kernel 4.0 I got an input lag on GDM when nouveau driver is used. G graphics Card is a NVidia Quadro K620.
Neither mouse nor keyboard reacts correctly/in time.
On the desktop (even with GNOME Shell) everything works fine again.
The problem only occurs when KMS is used. With nomodeset everything works fine.
Is this still an actual problem? AFAIK you've gotten much further.
This only happens on wayland