Most often when software suddenly goes from speedy to slow the reason is the memory requirements have moved from all in-memory to being large enough to page to disk. You may just have outgrown the memory you have available. Do smaller models still seem fast?
Another large contributor to speed is a graphics card. If your setup is using software emulation, that would be a drag on memory usage as well and could also put you over the edge. PTC typically uses either OpenGL (graphics card) or emulation.
Other than that, take a look at what processes are running and how much cpu and disk activity there is that might be taking attention away from what you want.