We have evidence this is an issue: we’re able to load the MSG much higher when using the Web Audio API without a MediaStream from getUserMedia. Lock contention profiling has been broken on a stock macbook pro (because of MacOS System Integrity Protection, that broke a bunch of dtrace probes), we have to use Linux or something to profile this. On Windows, when profiling, I see that in the locking stack, there is a symbol that hints at the fact that the mutex is known to have non-trivial contention (it’s a symbol like LockInternalSlowPath or something), which means it’s not using fast spinlocks or similar techniques to enforce linearity.

This is the bug where we remove the locks from our code. It's still going to lock in the APM function, this is going to be handled in bug 1404962.