Wow, who knew the answer to the vexing slowness problem was so simple? Someone in Feedback Hub did, that's who.
Problem description: "We’re working on a fix resulting in WSL users finding that File Explorer launch performance regressed after upgrading to Build 21354 and higher"
Solution (it's merely a wrong path!) is in Regedit, which you need to run as Trustedinstaller*:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\IdListAliasTranslations\WSL
"Source" there should be \wsl.localhost and not \wsl
*Several ways to do that, including: