A top class engine requires a JIT and that generally leads to being large and having high startup time. LuaJIT is pretty much the exception which proves the rule and 1. Mike Pall is a monster of a developer not unlike Bellard; and 2. Pall is on record (in discussions with other JIT people) that in his opinion you can’t bolt a JIT to an existing runtime and get a maximally efficient JIT. So you’d need someone of that class of skill to decide to make that their legacy.

And unfortunately, making a very fast interpreter-based JS engine is a tedious and human-power-heavy work, it involves a lot of work which can't be eliminated by better skills.