Tired trim-pump, obstructed hyd. lines, air-in-lines, and high resistance power connections at pump can all be causes...but most often, when a boat gets older, the seal/bushing area where the ram comes out of the cylinder, gets corroded and tight, causing slower movement from one side to the other. It's fairly common.
Clean connectors @ pump, check resevoir for fluid level, and listen for any difference between the sound of one pump vs. the other. If your boat stays out of the water, you can spray some lubricant like 6-56 on the trim-ram shaft when it's trimmed all-the-way up, then lower it and see if it speeds the action any...this will indicate seat/bushing situation that I referred to. Good luck.