I expect B3 and JS JITs in general to remain much faster, for the simple reason that speeding up compilation by 2x might be worth a 5% throughput loss for a JS VM (numbers made up, but you get the idea). But it is very unlikely such a tradeoff would be acceptable to LLVM, given the projects and corporations already heavily invested in it.

In theory, LLVM might support both a very fast compilation model and a slower and more efficient one. But supporting two complete codegen backends is a lot more effort.

And overall, codegen compilation speed has never been a priority for LLVM to anywhere near the extent that it is for JS VMs. It still doesn't have parallel codegen, while all JS VMs do. That shows the very different focuses on those projects.