I have 4 MK8 .. only tried one of them on a Geeetech printer (don’t buy those… :D). Had lots of trouble trouble until i lowered the speed way down (<30). I got annoyed and printed a modified version of the extruder from the Prusa Original and used a e3dv6 clone, now it works fine at 50mm/s.
My main printer is Ultimaker Original+ I never printed slower than 40 mm/s on it, my MPCNC parts are printed at 60-80mm/s.
So for me 30 mm/s is so slooooow 🙂