The game engine was written from scratch over a weekend, and has been built on incrementally since then. Performance is strongly tied to the browser's
canvas implementation, as drawing is where it typically spends most of its time.
All NPCs MUST DIE!™ is a highly experimental game, using highly experimental browser features. As such, there are some known issues with various popular browsers.
Browser compatibility notes (in order of how good the experience is):
Google Chrome 11 for Windows: Gameplay is smooth, with a framerate in the mid 200's. Audio for the entire browser drops out after about 5-10 minutes of continuous play, sometimes less, and sometimes in parts.
Internet Explorer 9: Gameplay is smooth, with a framerate in the mid-to-high 200's, sometimes breaking 300 FPS. No game audio whatsoever. Framerate inexplicably drops to ~30 FPS in the shop.
FireFox 4: Gameplay is smooth for early levels, with a framerate in the mid 80's. Occasionally decides that it has misplaced the canvas object, causing the game to freeze. Doesn't always pre-load images correctly, which is seen as a white flash before a level starts. Minor drawing offset problem, where a small white border can be seen around some game objects. Audio works correctly, however.
Opera 11: Performance is similar to FireFox 4, but without the minor drawing problems. However, once there's a lot of audio going on, the game gets a lot choppier than its average framerate would imply.
Google Chrome 11 for OS X: Performance is about the same as FF4, but with the same audio problems as on Windows.
Anything older: Upgrade your browser to something from the list above. You'll thank me for it.
Anything not listed here: Untested. Newer stuff - especially webkit-based browsers - tends to work better than older stuff.