Hello Guix!

If you’ve ever used offloading (or ‘guix copy’), you’ve probably noticed
that the time to send store items is proportional to the number of store
items to send rather than their total size.  Namely:

  guix archive --export coreutils

is fast, but:

  guix archive --export $(guix build -d coreutils)

is slow (there are lots of small files).

Running ‘perf timechart record guix archive --export …’ confirms the
problem: guix-daemon is mostly idle, waiting for all the tiny ‘guix
authenticate’ programs it spawns to sign each every store item.  Here’s
the Gantt diagram (grey = idle, blue = busy):