Did your computer support WebGL before hand?
I'm on Chrome 18 beta too and it has a feature where it will use a software renderer for WebGL if it's not supported, with the intention of making 3D stuff work. They've licensed one of the fastest software renderers in the world for it apparently, but it's still nothing compared to real hardware. The problem is we've benchmarked WebGL and found it to be 2-3x faster than canvas 2D in the past, because WebGL was always hardware accelerated. Now they're adding a software renderer for WebGL it's worrying because it might "usefully" switch to the software renderer when WebGL is not supported, which may be slower than the canvas 2D. Hopefully it won't affect many people because it won't take over if hardware-accelerated WebGL is available, but still, WebGL doesn't work everywhere... I'm going to try contact them to figure out if this will be a problem.
Meanwhile if you go to chrome://gpu, does it list everything as hardware accelerated at the top?