Keyboard Lag

I am experiencing keyboard lag since the day I updated my laptop to Debian 9. I use MATE and Keyboard options are default. It is annoying because logins and passwords are often transposed, even when first booting the system. Sometimes I type the login or URL or even this text box right now and the text shows up between one and five seconds later, often in a different letter order than I typed it.

This happens even when no other user applications are running. This does not happen when I boot my Debian 8/Gnome drive. What other settings besides Keyboard can I check out to get rid of this?

PS: I know you'll wonder: I need MATE because video never works for me under Gnome, which constantly and inexplicably consumes a third of CPU cycles in either Deb 8 or 9 on my laptop.

I have used MATE from time to time with Debian 9 and other distros and have not encountered this.

Try booting to a Live CD of Debian with a different DE and seeing where the problem exists in the live environment. If it doesn't it may not be a MATE problem.

You could also try installing a different DE or a window manager to your Debian 9 system and testing with it.

This last is a complete shot in the dark, but have you tried blowing out any dust and debris that might have collected under the keys with compressed air? I doubt that's the problem, but that option easy and quick.

I suspect a software problem causing a lag that just manifests itself in the keyboard.
That's what I would search for: What could be eating up resources?
Start with

htop
iotop
journalctl
dmesg

Hello! No lag when using a LiveCD. Just like when I boot from my old Debian 8 (Gnome) drive. Cleaned the keyboard (brush & vac). And I have been monitoring CPU and Memory with those tools and haven't found anything particularly resource intensive, once there was some firefox process holding 30% memory for hours but restarting eliminated that (without eliminating lag). Everyone must think I am just really bad at spelling...