Alt+Tab isn't slow, it's just that by default there's a 0.2 second delay, I imagine so that a quick Alt+Tab won't get slower by creating thumbnails of windows or altering window opacities or things like that.
Open CompizConfig Settings Manager (ccsm) (install if not already installed), open the Static Application Switcher plugin config, go to the Behaviour tab and fiddle with the value of Popup Window Delay.
It's 0.2 seconds by default which I found to be too slow. You can set it to 0 if you want it to be instant, or some slightly larger value as desired - after some experimentation, I settled on 0.05 seconds, which means that a quick Alt+Tab to switch to the next doesn't start hiding the other windows or opening the window switcher panel but just goes straight to the next application (this is particularly useful for comparing the contents of two windows without the flicker of the window switcher which I always used to get back when I worked on XP), but if I want to switch to any other window and want to see what I'm going to, there's no discernible delay.
