I'm guessing it's related to the number of assets of each type that you have installed on your machine. I have TS2010 on a laptop that only has built-in locos and the Trainz tab (F7) slides out in about a second. On my desktop I have several 1000s of locos and it takes 5 or 6 seconds to pop out. I think it might be due to Trainz having to build the list of assets and load all the thumbnail images when you click the tab. But that's just a guess.
Bob