I walked into the Arcade, long before the holidays patch, and have been treated to the most abhorrent framerates I've ever experienced in Spiral Knights. For the sake of real numbers, though, I will use the latest patch (as of 12/14/2011, 4:15 PST).

Let me set the stage:

Operating System: Windows 7 Ultimate 64-bit (6.1, Build 7600) (7600.win7_gdr.110622-1503)
System Manufacturer: INTEL
System Model: DX58SO
Processor: Intel(R) Core(TM) i7-920 @ 2.67GHz (8 CPUs), ~2.7GHz
Memory: 8192MB RAM
Available OS Memory: 8180MB RAM
Card name: NVIDIA GeForce GTX 470
Display Memory: 4064 MB
Dedicated Memory: 1248 MB
Current Mode: 1920 x 1080 (32 bit) (60Hz)
Sound: Speakers (3- Logitech G930 Headset)

Drive: C:
Free Space: 18.4 GB
Total Space: 76.3 GB
Drive: E:
Free Space: 385.2 GB
Total Space: 953.8 GB

(All straight from my DxDiag, with some editing.)

Very few spots give me trouble in Spiral Knights as far as framerates go. Sometimes Firestorm Citadel is weird, but that's nothing new. There was a recent update, though I'm not sure which, that causes me to suffer extremely low framerates in the Arcade. Not just in the main part, or any one place, but the entirety. It does get worse when there are more props, but in general I do not get higher than 15 frames-per-second with the current patch with the snow. On all other Haven maps I average around 50FPS with the snow. Before the snow I would say I average closer to 58FPS.

Before the snow lowering my graphics settings to Medium would double my framerate to 30FPS, while turning it to Low would double it again and put me back at 60FPS. After the snow patch, Medium actually shows zero improvement over High. ~13FPS, same as before. Low now gives me 30FPS. And this is all with render effects and compatibility mode turned off. Render effects only knocks High down a frame or two and doesn't change Low or Medium, same for compatibility mode.

While I know it's not game-breaking seeing as it's the Arcade of all places, it does reflect a little poorly on the game's behalf.