Yes I agree with Dan - Download and install Trainstore - Read the first few pages about installation of the manual tho.
Until you get used to it use the easy interface and always unstore everything if you add a new route.
The problem is normal with DC and Thames Mersey, the routes are so large MSTS installs everything from them. The more routes you have the more TrainStore will help in reducing loading times, plus it only loads the stock required for the activity.
If your downloading activities and updgrading DC to the lastest version RouteRiter is a must to check for missing stock. There are a couple of good tutorials on UKTS going though the procedure to update DC as a lot of people had prblems