Problems with slow moving power windows on my '88 Testarossa:

Don't know if this will help, but both my boxer windows were painfully slow and occasionally would stop halfway up. I bought a new set of switches off ebay and voila, problem solved

 

I don't know exactly what it is about the electrical systems with these cars.

They seem to be plagued with poor connection problems from the accessories to the engine electricals. - Try unplugging your motor connections in the door and cleaning them up. Sometimes the switches have poor connections, as well.

If you input a 12v jumper into the motor terminals and it speeds it up, you will know if its just that. I use a motorcycle battery.
That would be my first test.
Also, a dry spray lube, such as a graphite, on the window channel felt may help.

When I first got my TR, the interior and accesories were pretty ratty, and along the way I had to do both windows, as well.
I suppose that the former owner did not care to keep up the interior appearance, perhaps given the high cost of dealer maintanence.

The left window was so bad that the window would pop out of the track with a loud bang! when raising it unless you pulled it up by hand while hitting the switch. - Try that sometime.

That was a very puzzling problem, with an interesting solution, and at first, it seemed as if it was related to the slow moving window issue in some way.

 

"a dry spray lube, such as a graphite, on the window channel felt may help."

Anybody use silicon spray in the window tracks? How does graphite and silcon differ. All I know is that silicon is clear. Thanks!

 

We use a bit of both in the aircraft business.

The graphite is a microfine powder and works well quite a while. You would never see it in the channel felt. - Works great on locks, as well.
We can't use it around the panels because it is electro conductive.

The silicon is more of a lube and for me, doesn't seem to last.
Don't use it around anything that you ever intend to paint again.