This happens kind of off and on, but tonight was the worst I've ever seen it. I was running an anniversary instance with 3 characters, and one of them would sometimes get slideshow-level lag while the other two were running at normal speed. Tonight, after killing a mob I could alt+tab between characters and see it dead on two screens while the one was showing it still fighting at half health. It took a solid 60 seconds for him to get caught up to the other two. Same computer, same monitor, 3 instances from just the launcher, but somehow one ends up in a slideshow mid-fight. Has anyone experienced this or know what causes it? It's typically one character in particular, but sometimes it moves around, and it's never more than one at a time.
I had this happen to me for a while, never figured out really what was causing it, highly annoying to have your other boxes fine and one lagged to heck I agree. I did move my log files to a backup folder and start them again, if you are logging the character that's lagging may have a massive log file and writing to it may be causing lag on that one character. It did fix itself but was never sure if that's what was causing the issue