When Alt+Tabbed the client would experience significant FPS drop in Multiplayer. At glance it may not look like a big deal. But if you realise that in MP there is no real suspension and connected client still needs to run scripts despite the player not looking at the screen, it becomes a major issue.
I have constructed a simple test, 100 scripts spawned every 10 seconds and measured the difference between 1st script start and last script completion. It can take up to 4 seconds for the last script to finish when Alt+Tabbed. That is without real load. With little load it could reach 12+ seconds easily.
If player alt+tabs in MP this could mess client side big time especially in script intense mission. I would understand this happening in SP but not in MP.