I run in full screen mode and used to be able to run 7 or 8 screens at once and had no issue tabbing between instances, it worked smoothly and instantly. I think there has been some update to windows that has messed something up because when i do it now it takes ages to tab between instances even with just 2 running and most times after a couple of goes one will crash and the other will take a few minutes to recover. I updated to the latest drivers for my graphics card which helped a little.
About a year ago i had an issue where the game would not run at all due to a windows update that messed security settings up, the only way around it was a clean install of windows.