Yes the speeds you are looking to achieve are quite modest, so I would expect 12V so be sufficient, assuming they are 1.8deg motors.
Are you certain that the drivers are running at 1/16 microstepping and not some lower value? Check that the jumper settings you are looking at are for DRV8825 drivers, not the more common A4988. Also, some RAMPS boards have all the jumper positions shorted by PCB traces, so the jumpers don't do anything. If you run the motors at a slow speed, you can count the revs/second and work out whether it is what you expect at that speed.
Are you certain that the drivers are running at 1/16 microstepping and not some lower value? Check that the jumper settings you are looking at are for DRV8825 drivers, not the more common A4988. Also, some RAMPS boards have all the jumper positions shorted by PCB traces, so the jumpers don't do anything. If you run the motors at a slow speed, you can count the revs/second and work out whether it is what you expect at that speed.