I don't know if they allow tethering on prepaid, someone else will have to chime in with the answer to that.
Assuming they do allow tethering like they do with their postpaid plans...
It depends on whether or not your phone is on EDGE, HSPA,DC-HSPA or LTE, network conditions, etc.
Tethering is NOT a replacement for wired internet access and once you go over your tethering allotment you are going to get throttled to almost useless speed, usually 56k dialup speeds for the remainder of your billing cycle.
That said... on our T-Mobile plan I saw speeds like...
DC-HSPA - >= 15 down
HSPA >= 5 down
EDGE ... don't bother... 56k if that.
T-Mobile has so little LTE I can't test that and it's not in my metro.
Best case your going to get 15 or so down.
Pings will be all over the place on HSPA, anywhere from 80 to 800 ms depending, once again on network conditions.