Http/1.0 would require serial connections, and the "multiple files bad" absolutely applies. Bundling and minification into a single JS file was common, even required - and I would consider that a build step. Otherwise you are dealing with all your code in a single massive file.

On http/1.1 browsers would open 6-8 concurrent connections to fetch files simultaneously. At this point, code splitting had benefits.

Most webservers now run http/2 which can fetch multiple files at the same time over the same connection. I believe it is "virtually unlimited" and the initial connection setup - which is often the largest performance hit - only happens once. At which point code splitting has such little impact on the transport layer, that it is more perfomant than transferring and loading all the code.

https://blog.vespa.ai/http2/ Has an more details as well as some load testing against http/1.0, http/1.1 and http/2

There is also a limit to the number of files the browser can download in parallel, so if many files have to be fetched, they have to wait until the previous downloads are finished. This slows down performance even more

Yeah i programm a lot in python and typescript too. Even if i type something in python its still very weakly typed.

I recommend Pyright over Mypy if you don't mind it being owned by Microsoft. It has far fewer bugs, and if you do stumble on one, you don't have to fix it yourself because Microsoft's paid devs will fix it in a couple of working days (at least for the small bugs I've reported).

its like js but with extra steps WHY NOT USING TS then?

That sounds like spending a lot of extra effort just to avoid a little up-front effort.