When assigning buttons to a control, there are 3 settings for sensitivity (button repeating while pressing it)  for them:

Minimum setting = a single click trims a little bit - multiple presses can be needed to trim the aircraft to a good setting at the beginning, after that it can be quite good. No button repeat.
Medium setting = a single click adjusts the trim a bit faster. Slow button repeat. I find this the ideal setting, and use it for my Logitech Extreme 3D Pro joystick.
Maximum setting = a single click adjusts the trim a lot faster. Fast button repeat. Too fast IMHO.

Other than those settings, there could be a way of setting up how fast a button repeat occurs with the buttons through a game profile program (if it has one).