Without load, any stepper should run at least 500 RPM so just 3.5 is a bit odd, even under load. Also, the only substantial load on a printer should be acceleration, so reducing acceleration should help.
Raising voltage won't drop torque at all. But at 3.5 RPM you should have very close to maximum torque anyways, even when running 12V steppers at 12V ( < 5V steppers are recommended at 12V supply voltage).
Another one: did you adjust motor current? It's the current which gives you torque, not (or only indirectly) voltage. See [reprap.org]
Raising voltage won't drop torque at all. But at 3.5 RPM you should have very close to maximum torque anyways, even when running 12V steppers at 12V ( < 5V steppers are recommended at 12V supply voltage).
Another one: did you adjust motor current? It's the current which gives you torque, not (or only indirectly) voltage. See [reprap.org]