I was working on a 13" 2010 MBP that's severely misbehaving during Netflix streaming (the Silverlight plugin; I assume other things with Silverlight would behave similarly, but I don't know of any easy tests). The problem started abruptly a couple of weeks ago; the user said that nothing got installed at the time. Symptom: Open a video in Netflix and let it run for a minute; after a short while (few seconds to a minute at most), kernel CPU usage will spike to consume any CPU not being used by the plugin itself, resulting in the entire UI slowing to a crawl--mouse lag, video choppy, etc. Flash works fine, everything else I tested was fine; just Silverlight. Disabling hardware acceleration in Silverlight did not help. Everything is up-to-date; 10.8.3, Silverlight latest 5.1 update, FireFox 19 (problem also happens with Chrome). Nothing else running, reboots don't help, plenty of RAM, no kernel extensions installed, no obvious messages in Console (apart from messages relating to UI lag due to the kernel overload). I tried booting into Safe Mode, then back to clear the kext cache, which also didn't help. Removed Silverlight, reinstalled from a fresh download, no good. Only non-stock things that are installed are VMWare Fusion and some sort of GoPro camera video kit. I'm sort of stumped; I assume that Silverlight is conflicting with something else and causing the kernel to freak out, but I can't even find anything else to disable. I'm assuming it's not a Silverlight/10.8.3 conflict, or there'd be tons of Netflix users complaining. Any suggestions?