1.68A is pretty much for an A9488. I think it needs cooling i this case. The DRV8825 is capable of a little bit more amps.
You could use microstepping. There is less noise, but it will be even slower - you must use higher steprates.
The speed for function 2+3 is set when the fucnstion is activated. You can change to a higher value.
stepper1.setSpeedSteps( 2000 ); // Speed when moving home
stepper1.write(0);
Remember that setSpeedSteps needs the steprate in steps/10sec. You can use values up to 25000.
These drivers can drive your stepper very silently. But as far as I know only when using microstepping ( makes it slower ).