"The WebAudio API does provide a ScriptProcessorNode for JS processing, but it appears to suffer from the problems I described."

Yup. That is being worked on by W3C and the WebAudio committee as we speak. WebWorker based JS processing is something being considered.

https://github.com/WebAudio/web-audio-api/issues/113

http://lists.w3.org/Archives/Public/public-audio/2013OctDec/...

I am expecting this to be resolved in a few months. (hopefylly?)

If the threading issues are resolved, then real time audio rendering doesn't take that much processing power and can be easily done in JS.

Infact if there is nothing else contending for the EventLoop (like in this example of WavPot) the ScriptProcessorNode is able to meet the real-time guarantees pretty well, while still doing decent processing.

Also going forward, with Technologies like SIMD.js and asm.js more can be done within a single onprocess callback.