Pro tip: add `zmodload zsh/zprof` at the beginning of your zshrc and `zprof` at the end, open a new shell to see those pesky long-loading sources as a nicely formatted trace with timings.
When I was using MSYS2 (cygwin based, so with a slow fork) I ended up writing my own solution for a cute prompt and history logging to sqlite with a fzy frontend because the existing solutions were all made for Linux and assumed among other things fast forks.
> the tl;dr is to add zmodload zsh/zprof add the very top of your ~/. zshrc and zprof to the very bottom, then restart the shell. On startup, you will see a table with everything impacting your shell startup time.