It should not be slow, you start out at a low speed until you are away from Paddington then your speed picks up. The only thing I would ask is did you release the brakes fully.
Have you submitted a ticket for the problem, the only other thing I can suggest is make sure if there is a parking brake that it is also released.
I had brake on running and could only get up to 118. I'm a compete newbie to this game and to train driving sims so it's probobly me.