git-last-modified(1) slower than git-log(1)?

[Gusted <gusted@codeberg.org>]

Hi,

I'm working at switching Forgejo's implementation of getting the last
modified commits in a directory to git-last-modified(1). I'd expected
equal or better performance than the current implementation, but have
not yet been able to get this and I'm a bit puzzled as to why.

The current implementation of Forgejo (inherited from Gitea) works
roughly like this:
1. Run `git log --name-status -c --format=commit%x00%H %P%x00" --parents
--no-renames -t -z $OID -- :(literal)some/path`, the output of this is
quite complex and possible outputs more information than necessary.
2. The output of this is piped to some code to a parser and reconstructs
what commit ID last modified each file in the directory.
3. Via `git cat-file --batch` get each unique commits information.

With git-last-modified(1) (-z --show-trees --max-depth=0) this replaces
step 1-2, but is slower. I've isolated the degraded performance to the
fact that git-last-changed(1) takes more time to finish. So from my
perspective it does not seem worth it to replace the current
implementation with git-last-modified(1), and I would like to know if
I'm missing something here or if git-last-modified(1) possibly could see
a speedup?

The repository I'm currently using to evaluate the performance is
https://codeberg.org/ziglang/zig

Reproduction steps:
1. `git clone https://codeberg.org/ziglang/zig $(mktemp -d)`
2. cd to tmp directory.
3. `git commit-graph write --changed-paths`. As git-last-modified(1)
makes good use of the bloom filters.
4. `hyperfine 'git last-modified -z -t --max-depth=0
80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- doc/langref/' 'git log
--name-status -c "--format=commit%x00%H %P%x00" --parents --no-renames
-t -z 80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- ":(literal)doc/langref"'`

With as output:
Benchmark 1: git last-modified -z -t --max-depth=0
80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- doc/langref/
 Time (mean ± σ): 66.5 ms ± 0.6 ms [User: 60.6 ms, System: 5.2 ms]
 Range (min … max): 65.3 ms … 67.7 ms 44 runs

Benchmark 2: git log --name-status -c "--format=commit%x00%H %P%x00"
--parents --no-renames -t -z 80d06578ac66bce3aa0a21e9610cdb782b9a0593 --
":(literal)doc/langref"
 Time (mean ± σ): 26.2 ms ± 1.0 ms [User: 17.3 ms, System: 8.4 ms]
 Range (min … max): 24.3 ms … 30.1 ms 110 runs

Summary
 git log --name-status -c "--format=commit%x00%H %P%x00" --parents
--no-renames -t -z 80d06578ac66bce3aa0a21e9610cdb782b9a0593 --
":(literal)doc/langref" ran
 2.54 ± 0.10 times faster than git last-modified -z -t --max-depth=0
80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- doc/langref/

Kind Regards
Gusted