&gt; Significant parts of the functionality are written in JS.<p>Such as?<p>&gt; It&#x27;s often bounded by JS performance. JS performance depends on the JIT output, much less so the speed with which the JIT itself runs.<p>On what? Last I looked, even things like FB &amp; Gmail are largely bound by performance of C++ code. Sure, there&#x27;s a few hot JS functions high-up in the profiles (esp. in the large data structures they build up when initially loading), but by and large time is spent in C++ code, not JS.