emacs find-file extremely slow over sshfs+vpn

[Kim Barrett <kab@conundrums.ca>]

I'm trying to use emacs (29.1-1, also 28.2 and 28.1-4) on MacOS to edit files
on a remote machine via VPN and sshfs. (Mac-compatible emacs obtained from
https://emacsformacosx.com/builds.) Using emacs to read or write such files
takes a very long time, like 2-3 minutes. These are not large files. Obviously
this very usable.

It seems like the VPN is connected to the problem, since I don't see such a
delay when reading and writing files accessed via sshfs on a remote machine
that doesn't need VPN for access.

The network performance over VPN isn't amazing, but by itself it's nowhere
near bad enough to account for what I'm seeing with emacs. And I don't see
anything like that slowdown with other applications (including Aquamacs, a mac
port of emacs 25).

Searching the web finds various similar sounding complaints of various
vintages, but I'm not finding anything that matches what I'm seeing and has a
solution.

Any suggestions? I'd really like to switch to a recent version of emacs (I've
been using Aquamacs, but it's been stuck on emacs 25 for a long time). But
this problem makes that impossible for one of my main uses.