You have to realize that not only are performances of nacl probably going to improve with time, but more importantly this is not a port of the game in nacl, this is a port of the game, running in the dosbox emulator running in nacl (Dosbox emulate a whole old computer, which then load the game). Redoing such a game from scartcg in Nacl and getting rid of the emulator layer would have much better performances.

Another reason is explained on the website:

"Currently NaClBox is using far more CPU than it should. This is because the current version of SDL for Native Client has a bug that will crash the browser when you navigate away from it. To avoid that, I had to revert back to an older version of the SDL port that has high CPU utilization."

Those insisting that there's no way this will be another ActiveX would be wise to notice this. We're not even one day into the release and already there's a browser-crashing bug.

In addition to the other points, it should be pointed out many games had busy loops in them. Such games will always take 100% of your processor, except in some cases where the busy loop crashes because it manages to count too high and overflow something or other such crash bugs.