The client makes alt-tabbing a struggle. Takes an extra like 10 seconds or so for the screen to go from black to showing my desktop.
We test Alt-tabbing with all games and almost never get any results that are this long. However, it can also depend on the game, resolution, or other factors. Some games can take longer than others to recover from being minimized, and in some cases, not even being the focus.
Windowless Bordered and Windowed also drastically increase the speed of Alt-tabbing a game. Other factors can include if other programs or recorders are hooking into the game, etc.