bug#41646: Startup in Windows is very slow when load-path contains many entries.

[Nicolas Bértolo]

> So this is not specific to Windows, it's just that Windows has slower
> file access.  IOW, if load-path becomes significantly larger, the slow
> startup will show on Posix systems as well, right?

Exactly, but not nearly as bad. I profiled the same Spacemacs configuration in
Ubuntu 20.04 and startup takes 10 seconds, with 5 seconds spent inside openp().

Compare this to 40 seconds in total and 32 seconds inside openp() in Windows.

> Next question: are the 'wopen' calls coming from 'openp'? if so,
> perhaps we could first try a cheaper call, like 'chmod' (or its Win32
> API equivalent), and save the 'wopen' call if 'chmod' fails?  Did you
> try that?

They come from openp(), yes. I haven't tried it. I think the issue is related to
the IO architecture in Windows, so I don't expect major speedups, but it could
certainly help.

> My main concern with a cache is how to make sure it reflects what's on
> the disk, when files are added or removed.

That is my main concern as well. A good option would be to rely on package.el to
generate the load-cache.el file for a package when it generates autoloads. If we
can't find any of the files mentioned in the cache then we fallback to the
current load-path code. That is a simple solution that would take care of
addition and removal of files, I think.

Nico.