The slower the MG is, the better its combos will be (EX: lol knight mg is slow but has ai huge hitbox that can hit as high as ice fear lolwat). The Death MG has a series of wonderful combos, so of course it's going to be slower than the Toaster MG/Unicycle Bot. The Knight MG is about the same speed as the Death MG - but is obviously "better" than the Death MG in terms of DPM/size of hitbox.
Either way, you're going to have to deal with the MGs slowness, because you can't keep a low leveled MG and expect to do well at higher levels. You can, however, use any MG you want in PvP since all you'd really need are 10-enchant-slots (since weapons SORT OF balance themselves out to be more fair).
"He who fights and runs away, lives to fight another day!"