By way of comparison, Carakan (Opera's last JS VM) was under 10 man-years from start to shipping, including all testing resources, etc. (And yes, I realise man-

By way of comparison, Carakan (Opera's last JS VM) was under 10 man-years from start to shipping, including all testing resources, etc. (And yes, I realise man-years is a terrible way to judge this!) How much of Dropbox could they rewrite for something quicker than CPython in the time taken to write a JIT? That's the ultimate business question, and there's plenty of evidence to suggest that in general JITs have been comparatively easy versus a rewrite. As the article says, they have millions of lines of code. At the same time, at least from my knowledge of the languages, JS, Lua, and PHP are all easier to optimize than Python is (because you can do crazier stuff with Python's object model). JS and Lua also benefit from their incredibly small standard libraries which also means you don't need to potentially spend lots of time reimplementing libraries; CPython at least has most of its being implemented either in Python or through its C API which means if you have C API and licensing allows you can just pull out much of it from there.