A major feature just got merged in rustdoc: merged doctests. Currently, doctests are slow because they are all compiled individually. Merged doctests however are, well, merged into one file, compiled once and then each of them runs in its own process.
A good example to show how big the impact is: on libcore, doctests took 54s, they now take 13s (and 12s are spent on doctests which cannot be merged).
This feature will only run starting 2024 edition, but then, it'll run by default.
This is just the first step in our work on doctests. More improvements are on the way!