For some odd reason Spiral Knights is using (almost) 100% of my CPU usage. I've tried using the client I got off Steam and using the play button on the Spiral Knight website and I get the same problem with both ways of starting the game. I've looked through the forums the try to find a solution to this problem but only found others with the same problem.
Did you try enabling vsync?
( or posting in one of the already existing topics you found instead of opening another one of those :/ ?)