Performance regression on NFS with new manifest version

[Roel Janssen]

Dear Guix,

On our cluster, the following command took less than a second to run:
guix package --search-paths

At some point, the same command took 30 seconds to run.

So, I tried previous versions of the profile and found generation 15
returned the search paths very quickly (as before), and generation 16
returned the search paths only after about 30 seconds (the regression).

Looking into the manifests ($GENERATION_15/manifest and
$GENERATION_16/manifest), I noticed that generation 15 uses manifest
version 2, and generation 16 uses manifest version 3.

What has changed, so that it takes a lot longer to run the same command
as before?  (this is probably disk-related, because that is a known
cause for trouble on network-mounted stores..)

Thanks for your time!

Kind regards,
Roel Janssen