My experience with Inventor 2015 is, that when I work with large assemblies, pc very fast run out of memory.
Inventor does not empty cashe memory when i close asm and for that reason it start to work very slow.
So easiest way to work, is constantly restarting inventor and if it is very bad, restarting pc.
In picture below it is obvious that even model is closed, cashe memory is not emptied. If opening a new large asm it will quicly use free memory and begin to work very slow while eating bit by bit of cashed memory.
Never had this problem in Inventor 2013 so Autodesk please correct this