Well, the short answer is: hard to say. A lot of work is off-loaded on GPU's now days, and theoretically there should be no reason why talking to the video-card through a browser should be slower than talking to it through a portable executable (.exe). The reality of it is that there is far more sandboxing and verification that needs to be done since no one likes having their browser crashed out of the blue. As for performance of languages (JavaScript vs C or x86), naturally, you will always get more performance the closer to the iron you are but it's getting fairly blurred and compilers/interpreters are pretty damn smart these days.

I realize I'm not answering your question entirely, but what I read between the lines is you're wondering whether WebGL will be able to replace "native" applications, performance-wise in the future. The ball is in the air on that one, and it's about to be caught by Intel/AMD/nVidia. And I bet you they are already cuddling with the browser-developers (or at least rubbing their hands in glee).

I guess the quick and dirty test for the state of things right now is to simply check the CPU/GPU performance on this one and compare it with current top of the month here: http://pouet.net/prod.php?which=56871 (note that this is a 64KiB demo -- in comparison, the background picture of http://romealbum.com/ is three times bigger).

A more fair comparison would be: http://pouet.net/prod.php?which=56900 (also released this month, it seems)

Do let me know if you come to any kind of conclusion :)