&gt;Almost all of the lines of code are C++<p>Significant parts of the functionality are written in JS.<p>&gt;but how often is web performance bounded by UI performance?<p>It&#x27;s often bounded by JS performance. JS performance depends on the JIT <i>output</i>, much less so the speed with which the JIT <i>itself</i> runs.<p>The rendering engine may be a factor, but there HW acceleration also factors in. It&#x27;s far from being clear cut.<p>Media decoding already uses optimized assembler, nothing for the compiler to do there.<p>I&#x27;d suppose most of the gain would be in layout?<p>&gt;Seems unlikely, given the output of ICC is crippled for non-Intel x86 chips (which de-facto means AMD nowadays).<p>You can patch that out. I&#x27;d hope this Waterfox is doing that...<p>Even so, Mozilla provides a stub installer that downloads the right binary nowadays. It could check the chip vendor and get the appropriate binary.