This topic was moved because, while your car is a 5.0, it is still a SN-95 and shares more in common with the 96-04 crowd, than the Fox body platform.
With that said, the "slow down" issue is pretty common on older seats. There are a couple reasons. 1) the motor is just worn out. The motor just wears out after a while, needing to be replaced. 2) Broken gears/dry shaft-motor. The gears are known to break, and when the shaft dries out, it doesn't help things. 3) driver weight. I went to help a guy because his seat would barely work. I figured a quick tear apart, clean up, lube....and it would work. Well, when I got there, he was 280. The seat worked fine until he was in it, but with him in it, it barely moved. I still took it off, cleaned it up, but it didn't change anything.
Since you have the seats out already, the rebuild is actually pretty easy. There are some rivets that need to be popped, then put back, but a rivet gun from harbor freight with rivets can be had for less than $10. Pull the whole thing apart, look for rust, gunk or other crap stuck in the rail. I have seen melted M&Ms stuck in them..... Clean it all out, sand the rusted areas, or areas that are not smooth. I then usually use Rustoleum on the rails. The contact areas, I may go over with some very fine sand paper, to smooth it out, or even remove the paint all together in those areas. Grease it all up, and put it back together. Pay special attention to the gears, shaft, and rails. before you re-attach the motor, it should slide EASILY. If not, something is wrong. Add the motor. If it still barely moves, replace the motor. I get motors for $10-$15 at a pull-a-part type junk yard.
Good luck