--- Comment #2 from Michael Chang email@example.com --- Hi Jan,
Can you please test by disabling theme and with gfxterm and background ? It's simple to do that by editing /etc/default/grub and commenting out the GRUB_THEME=..
GRUB_BACKGROUND=/boot/grub2/themes/openSUSE/background.png # GRUB_THEME=/boot/grub2/themes/openSUSE/theme.txt
After that, run grub2-mkconfig -o /boot/grub2/grub.cfg
I think we need to check whether it's related to how grub's theme rendering and compositing things to target . And if you still experience slowness with gfxterm, I suspect it's more related with firmware as the flipping to front/display buffer is somehow slow in larger resolution..