On my Ubuntu MATE machine, switching applications with ALT + TAB was impossible, because it took nearly 2 seconds. The culprit appears to be an "UX improvement" gone bad: MATE renders a live screenshot of each running application inside the application switcher.

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Fixing this is simple:

• Open "Window Preferences"
• Either uncheck "Enable software compositing window manager" or check "Disable thumbnails in Alt-Tab"