Window previews (e.g. in overview) were being mipmapped (antialiased) in real time. This is generally considered a bad idea in OpenGL because most desktop GPUs can't regenerate mipmaps (recursively resampled copies of a texture) efficiently. So turn it off, just as we did in Compiz many years ago.
The measured performance win by doing this is a 20% reduction in mutter's render time.
Also note: The wallpaper thumbnails in the workspace switcher are never mipmapped (AFAIK) in gnome-shell, and always have jaggies. Yet nobody noticed those