… we’re doing better:

--8<---------------cut here---------------start------------->8---
$ guix gc -D $(guix build polkit --no-grafts)
finding garbage collector roots...
[0 MiB] deleting '/gnu/store/8vsfk8312m5p461wl4dq5lxjagfbwfhz-polkit-0.121'
deleting `/gnu/store/trash'
deleting unused links...
note: currently hard linking saves 72397.13 MiB
$ time GUIX_PROFILING="rpc reference-cache derivation-graft-cache" ./pre-inst-env  guix system vm gnu/system/examples/desktop.tmpl 
0.2 MB will be downloaded
 polkit-0.121  198KiB                                                                                1.6MiB/s 00:00 [##################] 100.0%
/gnu/store/wp3lv4xrh6vw79gnkyi5471c1l8j624n-run-vm.sh
Reference Cache:
  fresh caches:     8
  lookups:      17232
  hits:         14816 (86.0%)
  cache size:     784 entries
Derivation graft cache:
  fresh caches:    39
  lookups:      128402
  hits:         111336 (86.7%)
  cache size:     328 entries
Remote procedure call summary: 10796 RPCs
  built-in-builders              ...     1
  query-substitutable-path-infos ...     2
  build-things                   ...     2
  add-to-store/tree              ...    56
  valid-path?                    ...    59
  add-to-store                   ...   373
  query-references               ...  2416
  add-text-to-store              ...  7887

real	1m31.550s
user	2m7.107s
sys	0m0.637s
--8<---------------cut here---------------end--------------->8---

There’s still room for improvement though.

Ludo’.