Performance depends pretty much solely on whether the browser is hardware-accelerating the rendering. Old graphics cards might not be supported so you should say what the graphics card is on your different test machines. It's not a graphically intense game so I'd expect anything with a modern graphics card to get a solid 60fps. Also you should make sure you have the latest graphics card drivers since out of date drivers can cause slowness as well.
Also, do you have WebGL enabled? Where supported that can improve speed by 2-3x. You need to use the 'renderer' system expression to show which one is in use because WebGL may still not be supported and fall back to canvas 2d.
I get a solid 60 fps on Firefox 9, Chrome 16 and IE9 with latest drivers and a Radeon HD 6570.