>And it does this every time you spawn a shell.
It does not. The completion generation is on first start and whenever you run `fish_update_completions`.
(note that the importance of man parsing is often quite overstated, it only gets you so far, all the advanced completion stuff is hand-written)
Source: I'm a fish dev.
On NixOS, we generate the completions on behalf of users at install time, so they never have to see a slow startup at all. Fish makes it easy to do! Thanks :D