I have same model with no speed problems can go as slow as a snail or as fast as a cheetah. It is noisy when going too fast.
Mine is also set up on a frame. I never stand at it . I use a breakfast chair which has height adjustment and arms but no wheels. I can concentrate on one section before moving onto the next area.
I would try and take it for a service it could be dust build up somewhere. It may also need a little oil.
I love my set up.