the OS does use CI some, you'll really notice it if you have dashboard enabled though, that's the biggest offender. It also makes a HUGE difference in apps like Aperture, which are very very heavily reliant on CoreImage. They'll work if you don't have a CI GPU, but what that does is put *all* of those calculations off onto the CPU, which slows everything else down too.
But I suppose it only really matters if you know you've got a lot of CI-heavy apps.
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