Please see the screenshot to understand what I mean here.
Do you have prevented it from being updated automatically during the marathon? IMHO, this is probably a cause of slowdown.
If you want better client performance, try to disable every css animation you can find. (like on the trophy and flashing on back-to-tournament-button). I did some quick debugging and think that was the cause of a big memory leak in recent chrome version 42 and 43 on linux, +0.5GB +50% cpu after a few seconds.
I put the investigation on hold to newer versions because i lost faith in humanity while trying to navigate between all the (regression) bugs in the chromium bug tracker.
graphic animations are handled by the graphics card, but it's true that this is a cause of slowdown. But my problem was particularly on the chessboard. There is little or no graphics animations except the scoreboard. when there are 100 to 300+ registered players, it is a source of slowdown and cause of multiple 'mouse-slip'. Yes, the mouse-slip as the race of mouse slowed or stopped before the target square.
by graphics animations I mean webkit-animation