I see that you have layered a Buffer-style API on top of your per-sample code (api-buffer-effects.js), and I could still be missing something, but it looks like that just automates the call to process each sample individually --

    //api-buffer-effect
    self.effects[n].pushSample(buffer[i + n], 0);

    //api-generator
    out[i + n] = this.getMix(n) * this.mix + buffer[i + n];

This is very different from being able to process batches of samples within your effect or osc code -- even if the JIT inlines the functions of this.mix, you will still incur unnecessary setup cost.

Of course, without profiling this is just speculation.