I think maybe the underlying problem here is that some people somehow manage to write javascript inside .ts files and I haven't figured out how they do that. I doubt it though, I think there is no way. It always complains about lack of types, it wants me to add annotations. The promise of mixing js and ts is not real for me. It just defies belief for me that people actually want to write squiggly line, broken autocomplete type of code and have it compile a tiny bit faster compared to just having a single unified workflow where typing, build output and build information always matches and you don't have to guess and look up whether the build is already done, or maybe there was a build error or maybe this time one of the type checks that I ignored actually mattered for js also, not just for ts or maybe the error was actually not just a typecheck error but also a DOM api error and on and on it goes. To actually want to live in this world of uncertain, mismatched information and expect to be faster iteratively is not believable.
How is it not a problem for you that you make a change in a .ts file and then you are immediately in a world of uncertainty: Is the typecheck already complete? No clear progress on this. The js output is probably already here, but not sure unless you constantly want to check yet another terminal window. So can I reload the page yet? Oh no wait, a few more type errors popped up after all, so I can't yet. This all happens in under 2 seconds but those 2 seconds are uncertainty and slow you down far more than any wait for a normal sized typescript project where typechecking is done integrated with the build.