FWIW, as a long time user of tectonic I just started a project using typst yesterday, I'm really curious if typst will be able to do parallel downloading of packages than tectonic.

This is something which I have found can affect initial first time document builds since it can cause sequential downloading of a lot of small files sequentially.

The reason why it is difficult to parallelize or fix in tectonic itself is the way latex itself is parsed in a turing complete fashion. Importing a package can affect subsequent parsing. This makes it basically impossible to build a map of package dependencies without interpreting the entire document.

I'm curious to see how typst handles such things.

Yup. I use both LaTeX and Typst (although I prefer Typst when possible) and I haven't used anything other than tectonic for the past few years. It simply has everything you need included, and is pretty darn fast too.