Hi. There are many reasons for slowing. It can be a worn drive motor, but most commonly it will be a jam up or dry grease. You will need to investigate. Shine a light into the sides of the printer and have a good look for foreign objects for a start. Also look at the gears, and make sure the teeth are not full of dirt or bits of ribbon. This is very common. If they look dirty, get a toothpick, and scrape between the teeth until white again. It's time consuming, but this can really speed up a printer. The other thing to try is to get a can of WD40, or general lube spray and just squirt a small amount around the shafts that the gears sit on, and the head rail (stainless steel rail that the head slides back and forth along - just a couple of small squirts, don't drown it, or get it all over the head).
That's all I can think of. If it is still faulty, take it to a repair centre. It is putting a strain on the machine, and it probably will die badly if left like this.
Mark, November 2008