GRUB is extremely slow - 1 second per key input

Grub seems to have problems with high resolutions on certain systems. Setting grub to use a lower resolution fixed the input lag for me.

To do that edit the grub config file: /etc/default/grub

Look for the line GRUB_GFXMODE and change it to a suitable lower resolution for you system. E.g. in my case:

GRUB_GFXMODE=1280x1024x32,auto

Finally run update-grub as root to write the config changes.

Note: the resolution you choose must be supported on your system. To check them boot into grub, hit 'c' to get to the grub command line and type videoinfo to list the supported modes.

Ideally change the GFXmode as above so that it includes fallback resolutions if something goes wrong and add 'auto' (the default) at the end of the list. E.g. something like my_preferred_resolution,low_fallback_resolution,auto where each resolution is width x height [x depth]