Here’s a quick tip: if you have a monitor connected digitally to your Mac, but you have it powered off, your Mac may hiccup or stutter every second or two. The mouse and keyboard will be momentarily unresponsive, the clock will briefly pause, and if you’re playing video, the system will consistently drop a frame or two. It’s enough to drive a motion designer mad.
In my case, I had two monitors attached to my Mac Pro via DVI, but one of them was powered down. The stuttering made it difficult to work, and impossible to QC video.
The fix? Power up that second monitor, or physically disconnect it from the system.
My best guess is that the operating system is polling the display , possibly for HDCP compliance.