As the parent comment said, and I mentioned in my reply. JavaScript was just incredibly poorly optimized/not compiled and they applied 20-30 years worth of compiler research to make it significantly faster. You also had an alliance of every hyperscaler working on the tooling for a decade plus with help from all major hardware vendors to bring the best performance out of it. One driver of LLVM was Apple and WebKit which at one point was using LLVM for its JIT compiler so many improvements figured out in that period have also already been applied to LLVM.

LLVM already has decades of research applied to it to make it produce fast code, it will be incredibly challenging to even match its performance across all the targets it supports let alone improve on it in significant ways. It would be better to spend the time building an optimization pipeline for Zig itself and being more thoughtful about what code you send to LLVM versus trying to replace it wholesale.

Anyone that has ever played with dynamic languages like Smalltalk and SELF, or read their research papers.