Ways to improve performance?
Hi, is there any way beyond what's given in the client to improve in-game performance? I use an old desktop to run this game and would like to know if I could somehow further alter settings, such as changing game files. I would like to make it a lower resolution, even lower graphics potentially, etc.
Thanks.

The big ticket item in game is to go to the advanced options and enable compatibility mode. With that you lose translucent scenery but gain a performance boost. "Low" detail also disables the backdrops that can make some computers cry. The geometry detail for this game is pretty fixed so there isn't much in terms of tweaking that can help reduce the stress of 3D graphics. Similarly with the interface, many menus are fixed image sizes so playing below a screen size of 1024x600 is difficult as things get too cluttered. Also, it appears that you can't reduce colour depth to boost speed either as the game only uses 32 bit colour, registry edits for this yield no change. Though if colour depth settings are actually relevant to you for modifying performance you are likely running a PC that's a decade old.
Unfortunately for you Poseidon is probably on the right track with his advice. (He's a Dev doncha' know)

One thing I would suggest, is have a check to see what else is using your resources. Anti-virus software usually put a bit of strain on your machine (I'm using Avira Antivir, specifically because it's quite light-weight on resources). Other stuff like Browsers will take a chunk too so close them down if you're really stretched.
Are you specifically trying to reduce the resolution because of your screen-size etc? Or just thinking that a small resolution will put less of a strain on your system?
It sounds like you have tried the various options provided in the Options menu.
If it's an older desktop it most likely also has an older graphics card or onboard graphics controller. For a relatively small amount of money ($20-$30) you could get a better video card that would significantly improve performance not just for Spiral Knights.
You'll want to make sure you get the right type of adapter (PCI, AGP, PCIe).