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