I'm just doing a trial run of youtrack, liking it so far, it's great.
I did notice a serious performance drop when using the Compact or the Detailed view.
If i'm in the Tree or One-line view everything is fine and responds well, but as soon as I switch to either the Compact or Detailed view everything becomes very sluggish, some of the controls take over a second to respond, particularly the different fields for each issue. Scrolling is also pretty unresponsive.
I'm testing this on a Mac (2.8ghz core 2 duo) using Omniweb.
I'm on Youtrack 4.2.2