Backing the knurled screws out on the air fittings will increase the speed for the pot up/down action.
Your problem might be in the air solenoid, rather than the air cylinder. Does your lubricator bowl at the machine's air hook up have oil in it?
In the past, I've resolved issues like this by disconnecting the machine's air supply, removing the inlet air line from the solenoid, adding a few drops of light oil directly into the inlet tube, reconnect solenoid inlet tube, and reconnect machine air.
After that, go to the ATC Manual page and fire the pot up and down a few times.
Removing the solenoid valve, disassembling it, and cleaning the spool and bore will achieve better results, but make sure you insert the spool the right direction upon reassembly.
If that doesn't help, you can try adjusting the throttle valves on the up/down cylinder. Obviously, back them out slowly, and only back them out enough to achieve smooth operation.
If you have to open the throttle valves so much that the pot slams either way, you'll have to pull the cylinder off, disassemble it and clean the piston and bore.
The relatively small problem you're having now isn't worth trashing pots and/or the up/down cylinder.