Since the game is unplayable I thought I'd come see the forums to see if anyone would join me in speculating about this memory leak.
I don't usually use java, but I'm pretty sure it uses some kind of automatic garbage collection, which would seem to reduce the possible sources:
1. Something is not allowing references to die.
2. Something is being loaded or allocated repeatedly.
3. JRE bug.
(1.) is probably the most interesting to think about, since one can imagine some kind of temporary resources that are built in a usually self-contained area of the program that have been given a new lease of life by inadvertently being stored in some persistent part of the program. I suspect the UI's menus myself, since that's were the lag was absolutely the worst, and seemed to get worse when I was playing last night.
The steam JRE is also pretty old at this point (as of writing I get 1.6.0_07), so they could have uncovered an exciting bug in that.
Most people seem to think it is just a server issue, let alone realise it is something java related. I dont think there are enough of us on here that can code, or have some knowledge of how to.
I think you are probably correct as i can get java to increase very quickly simply by switching between mission lobbies. Id guess the game is loading a future feature on each level (eg danger room UI) and this feature is stopping the game from clearing memory properly.
all we can do is wait and speculate.
either way temporary workaround:
http://forums.spiralknights.com/en/node/56235