While 3 mH is not great, neither is it that bad. With your setup I would expect 1000mm/minute or better, possibly even 2000 mm/minute.
When you state that 500 mm/minute is the best that you can get, do the motors stall or simply refuse to go any faster? If stalls are the issue, try running the motor with it not mounted to the machine and see if it will go faster unloaded.