Second, JavaScript already executes quickly. Aside from arithmetic operations it has now reached performance parity to Java and highly optimized JavaScript (typed arrays and an understanding of data access from arrays and objects in memory) can come within 1.5x execution speed of C++. At this point all the slowness of JavaScript is related to things other than code execution, such as: garbage collection, unnecessary framework code bloat, and poorly written code.
That being said it isn't realistic to expect measurably significant faster execution times by replacing JavaScript with a WASM runtime. This is more true after considering that many performance problems with JavaScript in the wild are human problems more than technology problems.
Third, WASM has nothing to do with JavaScript, according to its originators and maintainers. WASM was never created to compete, replace, modify, or influence JavaScript. WASM was created as a language ubiquitous Flash replacement in a sandbox. Since WASM executes in an agnostic sandbox the cost to replace an existing runtime is high since an existing run time is already available but a WASM runtime is more akin to installing a desktop application for first time run.
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.
What I do know is that some people complain about long compile times in their code that can last up to 10 minutes. I had a personal application that was greater than 60k lines of code and the tsc compiler would compile it in about 13 seconds on my super old computer. SWC would compile it in about 2.5 seconds. This tells me the far greater opportunity for performance improvement is not in modifying the compiler but in modifying the application instance.
