Squarebody power windows and tailgate glass are notoriously slow. Going with the relay harness will fix that for the doors. I don’t know if they have that for the gate.
However the gate has other issues that can make it slow and cause the motor to drag or hit its internal circuit breaker and quit. Main issue is drag. The window has tracks that the regulator has rollers that slides in those tracks as it goes up and down. The tracks get gummy, dry or crusty and the rollers can stop rolling and start sliding. Without any lube the rollers will stop rolling and start wearing one side and create a flat spot.
So you need to pull the panel off of the inside of the gate to access the regulator. Once inside you can get to the rollers and tracks. You might need to move the glass to get to those parts. In that case the safety switch will not allow the window to move with the gate open. Instead remove the cable from the motor at the lower corner on the driver side of the gate. It unscrews like a Speedo Cable. Then take the end of the cable that spins and chuck it up in a cordless drill. Now you can run the window out enough to access the slides better.
Be careful with the glass out and the gate down. It’s pretty easy to break the window with it out like that if you lean on it.
Grease everything up good and run it in and out with the drill to make sure it works smoothly. Reassemble and give it a try.