Does it have an option to set the amount of video memory in BIOS? Linux in general is very good at utilising memory, so it makes a huge diference if there is plenty of it.
Most desktops with integrated graphic cards have that option, that would leave more memory to video card even when Linux is running out of it. Would make the overal performance of Linux slower but chances are the update of the video would be a bit faster.
I do not have any of the RPI so i can not test this, sorry bro.
There is another posibility to test, but requires another computer. SSH from the other computer and start Linuxcnc, that way the GUI will be on the other computer.
Did you check TOP to see how and what process is slowing things down?
P.S. Sorry for the Polish, my bad. You are in Slovakia, or as i like to call it the country with most beautiful women on earth. Been in Bratislava for 3 weeks, came back with some minor PTSD.