One write per frame would potentially be not so bad, that's true. I wonder though if you don't need to do some security measures there that could end up as sign

One write per frame would potentially be not so bad, that's true. I wonder though if you don't need to do some security measures there that could end up as significant overhead (for example, JS engines have some anti-heapspray techniques that do things to typed arrays - transferring large arrays between the NaCl sandbox and the JS sandbox might want similar things). But maybe not. Regardless, I think there are more suitable technologies for codecs on the web: A lot can be done in WebGL shaders (Broadway is starting to work on that), and proposed web standards like WebCL and RiverTrail look promising too.