I have a 290FX Powerquest, with what I can tell are factory installed Bennet Sport Trim tabs.
Both tabs move full range. The Port side moves drasticly slower that the Stbd.
I have checked fluid condition is good.
Whats common on these.

Call Bennett directly.
http://www.bennetttrimtabs.com/

They SET THE STANDARD for customer service. They don't care if your tabs are 25 years old, they will help you through any issue you have.

The furthest trim tab from the hydraulic pump can move slower - just saying as this is normal but in your case maybe not so. Make sure your line does not have a fluid leak or sucking air ( the slower moving tab side)

Is the issue on extend, retract, or both with respect to the cylinders?

Bup is on target, if one run of tubing is much longer than the other side, the one with the longer run can run slower. It does tend to even out when the boat is running and there is back pressure on the trim Tabs from the water flow.

If this is the case putting a longer run in a coil on the short side will even things out.

Tom McGow
Bennett Marine