This was touched on in the video a little bit—essentially, the TypeScript codebase has a lot of polymorphic function calls, and so is generally hard to JIT optimize. JS to Go therefore yielded a direct ~3.5x improvement.
The rest of the 10x comes from multi-threading, which wasn't possible to do in a simple way in the JS compiler (efficient multithreading while writing idiomatic code is hard in JS).
JavaScript is very fast for single-threaded programs with monomorphic functions, but in the TypeScript compiler's case, the polymorphic functions and opportunity for parallelization mean that Go is substantially faster while keeping the same overall program structure.
The rest of the 10x comes from multi-threading, which wasn't possible to do in a simple way in the JS compiler (efficient multithreading while writing idiomatic code is hard in JS).
JavaScript is very fast for single-threaded programs with monomorphic functions, but in the TypeScript compiler's case, the polymorphic functions and opportunity for parallelization mean that Go is substantially faster while keeping the same overall program structure.