Dear Lazyweb,

Some time in the last few months, video playback performance on my iMac has started to suck. Most videos now periodically pause for 5-8 seconds (video frozen, audio continuing). With many videos, this happens about every 30-60 seconds. With some, it's fine. On a "bad" video, if I skip back to just before the pause-point, it plays fine. If I skip back too far before the skip point, the pause happens at very close to the same spot. Sometimes it pauses a bit longer, and then audio stutters too.

This happens in Quicktime Player, Quicktime Player 7, and iTunes. Many of these are old videos that used to play fine. SD and HD.

Here's an example from today. It doesn't stutter when played full-screen HD on Youtube, but does if you download it. In Quicktime Player 7, this mp4 stutters at 00:02-09, 00:23-26. 00:46-50, 01:05-11, 01:25-32, 01:47-54, etc. Same spots every time if I don't rewind. In Quicktime Player X, it stutters slightly less often and for slightly less long -- 00:16-19, 00:35-36, 00:40-42, 01:19-22, 01:40-44, 02:04-06, etc. -- but again in the same spots every time. iTunes is similar.

Unfortunately, a bunch of things have changed on this machine over the last few months, and I'm not sure exactly when this problem began: I've upgraded from 10.9.1 to 10.9.2; replaced the RAM; replaced the internal drive. So there are several things that could have been the cause. It could be some new stupidity in the Quicktime library, or a disk performance problem, or... well, maybe it can only be those two. But I'm not sure how to test it.

Though the machine generally feels much faster than it did before that RAM upgrade.

Stats:

iMac 27" Mid-2010
2.93 GHz Intel Core i7
32 GB 1333 MHz DDR3
ATI Radeon HD 5750 1024 MB
HGST Deskstar 4TB 7200 RPM SATA III 6Gbps, 64MB Cache
FileVault on

Any idea what to test? I don't particularly understand how to interpret the output of iotop or fs_usage.