Searching nightly docs is inconsistently slow #146048
Description
Prior to the rustdoc search overhaul in #144476, it was typically the case that searching the nightly docs or nightly-rustc docs would be slow once per day (presumably when reloading the new docs), and any searches after that point would be basically instant (presumably because the index was fully downloaded and cached).
After the overhaul, it is now the case that most doc searches incur an unpredictable multi-second pause before returning any results, and continue to show a distracting throbber for several seconds after results are shown.
There doesn't appear to be any way to “preload” the full search index, which the previous implementation would do automatically.
This results in a much more frustrating experience, because most individual searches are now slow instead of fast.