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Ccache is working now and as expected reused in the PR once the first commit builds, see for example this build log:

+ echo 'ccache build summary:'
ccache build summary:
+ ccache -s
Summary:
  Hits:              72 /   72 (100.0 %)
    Direct:          72 /   72 (100.0 %)
    Preprocessed:     0 /    0
  Misses:             0
    Direct:           0
    Preprocessed:     0
  Uncacheable:        6
Primary storage:
  Hits:             144 /  144 (100.0 %)
  Misses:             0
  Cache size (GB): 0.01 / 0.51 (2.01 %)

This probably saves 3-5 minutes to the build time, see this PR commit build logs: https://app.circleci.com/pipelines/github/scikit-learn/scikit-learn?branch=pull%2F30350. First ones are not cached and doc build takes ~16 minutes, last one is cache and take ~12-13 minutes.

I also used -j3 to stick to the default ninja parallelism rule as mentioned in #30333:

• 1 core -> 2 ninja process
• 2 cores -> 3 ninja process
• n_cores -> n_cores + 2 ninja processes

I tried -j3 vs -j2 it does not seem to make a difference time information is about 1 minutes when fully cached (which I would say is most of the cases):

real    0m57.945s
user    1m40.114s
sys     0m11.052s

Thanks @lesteve