LLVM is bounded by what its IR can express, and what it can express is constrained by what is portable and what it can prove. There is a reason `ffmpeg`, OpenSSL, OpenBLAS, glibc's string routines, LuaJIT's interpreter, etc. all still leverage hand-written assembly.

As for the editor, even if it isn't necessarily optimal assembly (I have no clue), a reduced feature set and relying on OS APIs instead of frameworks will still probably result in something which feels faster than Electron programs like VSCode.