1) Turn down your graphics. Hit esc (at least that's what it is by default), and turn down your graphics to low. Then, go to advance settings and hit "Compatibility Mode." This basically reduces/removes a lot of the extra sparkle and shine. Some things will look a little weird, but it will reduce a lot of lag. Then, check the option called "Cull Transients." This makes so all the special pretty effects that happen off-screen don't render. This reduces a TON of lag, but beware, because sometimes you wont see Bomb detonations or hazards give their warning flashes before they activate.

2) Close anything that isn't Spiral Knights. The exception would be something like Steam.

3) Set Java Priority to High. Open up Task Manager (Ctr + alt + del), and then click the Processes tab. Find Java, right click, and then set Priority to High. In my experience, it reduces your frame rate a little bit, but makes it so you get less "Why is everyone walking through walls?" moments.