Assuming they are carbide cutters, I would run a 6mm 2 flute end mill in aluminium at 24,000rpm, 1500mm/min 2mm deep and slotting or full-width cut. In hard wood something like 4000mm/min.
But you must clear the chips so air blast at a minimum really.
If your figures used to work and have now stopped, something is not right - maybe a seized bearing or linear guide?
It may be a failing drive - is it one or both axes that lose steps?
If one it may be possible to swap the connections to verify if the motor is failing or the drive is failing.
Is this an all-in-one drive board - all three axes on one board, I have not heard many good stories about them. Most builds and upgrades go for separate drives running from a breakout board.