I've been noticing this for the past week or two (can't say when this started, I just noticed it recently). At the moment I'm using r11180, and whenever I'm downloading a big torrent (lets say over 4GB) and the swarm is healthy (allowing me to download at the highest speed possible), most of the times when I try to bring Transmission to the top it just locks up into spinning ball, I have to wait a minute or so for it to come. I've checked Console.app, nothing shows up there, I've also tried to see if the % CPU goes from Transmission goes nuts when the problem is happening, but it remains @ 4~5%.
I'm trying to see if by placing a download cap it gets fixed, with my current speed I should be able to manage 600~650 KB/s, or somewhere around there). First tried 650 KB/s, then 600KB/s, it kept locking up. Now I have it @ 590 KB/s and it might seem that the spinning ball is gone.
Also I noticed that when it locked, after it went thru the speed was at the roof (which is probably happening because it's still calculating and it was locked up) and then goes all the way down to 0, then gets back to where it should be.
Not sure if this report is a bit vague.