It's still pretty unfinished at the moment. But you can make a fairly big map, just not with terrain. The arrow I have drawn on points to the start marker so you can see the size of the player in comparison. Still getting mostly 60 fps even though it's fairly polygon heavy. It does slow down when you go beyond the normal borders allowed, but that is not a performance issue, that is just something weird
Not sure how it will go with enemies and such populating the map though, that is still unclear. Initial tests don't look horrible, but do slow things to about 40 fps (with 30+ zombies in same spot).
Edit
I forgot about the speed question. Not sure it can be altered as yet, but you can alter the behaviour while in game test mode.
It' still quite early though and this has only recently been added. I've not really trying playing with AI much as yet.
SPECS: Ryzen 1700 CPU. Nvidia 970GTX. 16 Gig Memory. Win 10.