GNOME bug 504729 suggests that switching with alt-tab, while using compositing, is too slow. This is because all the images of the windows are scaled on the client side before the window is displayed.
There are two possible answers to this problem.
Firstly, we can check for key release while scaling is happening, and if one is received, abort scaling and simply switch to the next application.
Secondly, scaling can be faster if it’s done server-side. This is possible but apparently there’s a common bug that it hits. Fixing this would also mean that we got to have animated previews easily.
I think the first solution should be added in any case, and the second should be added when it’s possible. I can add the first solution; I’m not sure I know enough to fix the second.
Photo © L. Marie, cc-by.