I don't think there is any disagreement about the speed. NaCl will be faster than Flash and other options.
But consider that Alchemy will run in 98% of web browsers (because that is how much range Flash has), while NaCl will only run in Chrome. So there is a difference of about 4X in market share.
I'm not saying Alchemy is a perfect solution. Each option in this area (Alchemy, NaCl, JS) has different advantages and disadvantages.
And, recently, started with NACL.
Games are extremely performance-intensive. Its simply crazy to pick a platform that is any factor slower when you have a faster alternative to hand.
NACL is a whole new level of possible. I think its going to succeed technically where Flash and plugins failed.