60 knots groundspeed is the usual maximum speed for a high speed exit.
At busy airports, as an example landing LAX 25L, they prefer you to turn off the high speed taxiway as it gets you off the runway quicker so the guy behind you doesn't have to go around. The time taken to slow to 10 knots for a turn onto a normal taxiway and the cross the hold short line can be significant in the case of a heavy like a 747.
