rather slow 'git repack' in 'blob:none' partial clones

Hi,

here are trace timings of running 'git gc' in a "normal" and in a 'blob:none' partial clone:

  $ git clone --bare https://github.com/git/git git-full.git
  $ GIT_TRACE_PERFORMANCE=2 /usr/bin/time --format='elapsed: %E  max RSS: %Mk' git -C git-full.git/ gc
  10:35:24.007277 trace.c:487             performance: 0.001550225 s: git command: /usr/local/libexec/git-core/git pack-refs --all --prune
  10:35:24.044641 trace.c:487             performance: 0.035631270 s: git command: /usr/local/libexec/git-core/git reflog expire --all
  10:35:24.061070 read-cache.c:2315       performance: 0.000008506 s:  read cache ./index
  Enumerating objects: 305283, done.
  Counting objects: 100% (305283/305283), done.
  Delta compression using up to 4 threads
  Compressing objects: 100% (75016/75016), done.
  Writing objects: 100% (305283/305283), done.
  Total 305283 (delta 227928), reused 305283 (delta 227928), pack-reused 0
  10:35:32.604546 trace.c:487             performance: 8.555651283 s: git command: /usr/local/libexec/git-core/git pack-objects --local --delta-base-offset objects/pack/.tmp-2946975-pack --keep-true-parents --honor-pack-keep --non-empty --all --reflog --indexed-objects --unpack-unreachable=2.weeks.ago
  10:35:32.680597 trace.c:487             performance: 8.633068356 s: git command: /usr/local/libexec/git-core/git repack -d -l -A --unpack-unreachable=2.weeks.ago
  10:35:32.683130 trace.c:487             performance: 0.000959377 s: git command: /usr/local/libexec/git-core/git prune --expire 2.weeks.ago
  10:35:32.684401 trace.c:487             performance: 0.000180173 s: git command: /usr/local/libexec/git-core/git worktree prune --expire 3.months.ago
  10:35:32.685730 trace.c:487             performance: 0.000263898 s: git command: /usr/local/libexec/git-core/git rerere gc
  10:35:33.514816 trace.c:487             performance: 9.511597988 s: git command: git -C git-full.git/ gc
  elapsed: 0:09.51  max RSS: 358964k

  $ git clone --bare --filter=blob:none https://github.com/git/git git-partial.git
  $ GIT_TRACE_PERFORMANCE=2 /usr/bin/time --format='elapsed: %E  max RSS: %Mk' git -C git-partial.git/ gc
  10:35:47.637735 trace.c:487             performance: 0.000872539 s: git command: /usr/local/libexec/git-core/git pack-refs --all --prune
  10:35:47.675498 trace.c:487             performance: 0.036246403 s: git command: /usr/local/libexec/git-core/git reflog expire --all
  Enumerating objects: 188205, done.
  Counting objects: 100% (188205/188205), done.
  Delta compression using up to 4 threads
  Compressing objects: 100% (66520/66520), done.
  Writing objects: 100% (188205/188205), done.
  Total 188205 (delta 119967), reused 188205 (delta 119967), pack-reused 0
  10:35:50.081709 trace.c:487             performance: 2.402625839 s: git command: /usr/local/libexec/git-core/git pack-objects --local --delta-base-offset objects/pack/.tmp-2946990-pack
  10:35:50.100131 read-cache.c:2315       performance: 0.000009979 s:  read cache ./index
  10:37:04.973541 trace.c:487             performance: 74.885793630 s: git command: /usr/local/libexec/git-core/git pack-objects --local --delta-base-offset objects/pack/.tmp-2946990-pack --keep-true-parents --honor-pack-keep --non-empty --all --reflog --indexed-objects --exclude-promisor-objects --unpack-unreachable=2.weeks.ago
  Removing duplicate objects: 100% (256/256), done.
  10:37:07.482791 trace.c:487             performance: 79.804973525 s: git command: /usr/local/libexec/git-core/git repack -d -l -A --unpack-unreachable=2.weeks.ago
  10:37:07.549333 trace.c:487             performance: 0.008025426 s: git command: /usr/local/libexec/git-core/git prune --expire 2.weeks.ago --exclude-promisor-objects
  10:37:07.552499 trace.c:487             performance: 0.000362981 s: git command: /usr/local/libexec/git-core/git worktree prune --expire 3.months.ago
  10:37:07.554521 trace.c:487             performance: 0.000273834 s: git command: /usr/local/libexec/git-core/git rerere gc
  10:37:10.168233 trace.c:487             performance: 82.533331484 s: git command: git -C git-partial.git/ gc
  elapsed: 1:22.54  max RSS: 1891832k

Notice the ~9s vs. 82s runtime and ~350M vs. 1.9G memory consumption increase. What's going on here?

Also note that that second 'git pack-objects' invocation doesn't show any progress for ~75s.

FWIW, doing the same in a 'tree:0' partial clone is fast.