I'll bite.

I think the "fear" is that once a non-web weary designer get's going with this there will be absolutely no regard for performance impact since it will render fine on what ever pimped up work station is in use.

Meanwhile on platforms with lesser hardware where the browser does not have a full hardware render pipeline the performance will likely be even worse then flash.

In fact, with the ferris wheel demo Safari eats 10% CPU time, Chrome 30%, Firefox 50% on a single core. System: OS X 10.6.8, GF 9400M.

Next thing to happen: "Disable javascript / css to save battery life on your device". :)