This is something you might want to consider having a consultant take a closer look.
You should start by looking at the task manager when this happens, is all memory being used? are all processors being used?
A few tips
- Make sure the application opens on a blank page so no objects are calculated when the user first logs in.
- If the application is just too big to open on your current server you might want to consider adding more memory to it, building a smaller app with sumarized data or showing less information.
- Make sure all graphs have been built properly, they don't use calculated dimensions, they don't show millions of rows and use calculating conditions.