Debian

Well, for start i would look at top a bit during lag, to see any waits happening and where does the system spend most time.

High sys values are not good indicator in most cases.

Next, one should always try to examine what has changed in the video driver stack.

Can you show output of following commands as root or using sudo on both versions of operating system ?

Code:

lspci | grep -i vga
glxinfo | grep -i renderi

Perhaps more when we determine the model and make, like modinfo and lsmod

Can you confirm that you are running operating system exclusively from medium/memory, it is not installed on disk(s) ?

Regards
Peasant.