I think the "rated" supply is relative to the reference voltage at the driver, which is typically 1.9V or thereabouts.
Your stepper is a typical NEMA17, which should work just fine off 12V. An ATX power supply should provide plenty of current. I believe the 2.5A is the total current... in single-stepping mode this flows through only one winding at a time. In micro-stepping mode, it proportionately flows through the two coils.
How were you controlling the speed? And what was your expectation of the maximum speed? If the driver is configured for 1/32 micro-steps, the motor will turn at 1/32 of the step rate. There are also limits to the acceleration/deceleration rate.
Your stepper is a typical NEMA17, which should work just fine off 12V. An ATX power supply should provide plenty of current. I believe the 2.5A is the total current... in single-stepping mode this flows through only one winding at a time. In micro-stepping mode, it proportionately flows through the two coils.
How were you controlling the speed? And what was your expectation of the maximum speed? If the driver is configured for 1/32 micro-steps, the motor will turn at 1/32 of the step rate. There are also limits to the acceleration/deceleration rate.