Hello everyone!
I ran across this issue with Boulder Dash II using Mame 0.289 as well. Down and to the right seem to move, pause, move, pause, move. The game is unplayable since he needs to be able to run past two stacked boulders heading downwards and go faster than the falling boulder! Up and to the left work just fine. I modified the X and Y sensitivity, return to 0 settings, no change. I flipped the X and Y axis, and while they keys are reversed, it's still right and down that have a pause. I changed the system setting for key repeats. I also mapped the keys to IJKL instead of the arrow keys, still happens.
I didn't see a post about this on MAME Testers, but I'm likely missing it. I didn't see an active github issue post either, and was thinking about posting it there. This slowdown doesn't happen with AppleWin, so it seems to be a way that MAME emulates the joysticks. Did anyone find a solution, or have any tips on fixing this? I'm happy to make an account and post this to MAME Testers (again?) if not.
My best,
Doubles