That switch is simple. Two conductors. Follow the cable down the lift cyl and find the plug, make sure it's fully seated and not damaged. Check operation of the switch with ohm meter. The switch is adjustable and can sometimes loosen up and not be engaged by the mast. It's either closed or open. The truck should slow way down with mast above free lift. The speed that the truck travels above free lift can be adjusted with a handset.

Got the exact same problem. While driving and raising the mast is will slow to a crawl when it hit the height switch.

one of the first thing I would do would be to reconnect the height limit switch and check the wiring and operation of that switch. I don't know how it was designed in your unit, but if I was designing that circuit, I would have "failed open circuit" switch and wire to be the safer default of not going fast if failed open, rather than slowing down ONLY when switch and circuit are closed.