Build on CI is getting slower

[user202729]

Steps To Reproduce

https://github.com/sagemath/sage/actions/runs/18178700275/job/51750086704 takes 22m for the build step. some weeks ago https://github.com/sagemath/sage/actions/runs/18068442370/job/51414615437 it only take 6m.

also note that in https://github.com/sagemath/sage/actions/runs/18068442370/job/51414615437 the cache size is shown to be only 60MB, this feels unreasonably small, but it is fast anyway.

speaking of which "Post Cache conda packages" shows

Post job cleanup.
/usr/bin/tar --posix -cf cache.tzst --exclude cache.tzst -P -C /home/runner/work/sage/sage --files-from manifest.txt --use-compress-program zstdmt
Failed to save: Unable to reserve cache with key Linux-conda-ca64ad1393d9754038da5cfa8e0a8e661f489ecd[2](https://github.com/sagemath/sage/actions/runs/18178700275/job/51750086707#step:35:2)a9dc30e8cdcfad52437f948, another job may be creating this cache.

not sure if it's relevant.

Expected Behavior

Actual Behavior

Additional Information

No response

Environment

• OS:
• Sage Version:

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[user202729]

actually some of them are fast. Are they just random?

https://github.com/sagemath/sage/actions/workflows/ci-meson.yml?query=is%3Asuccess

[user202729]

maybe it depends on number of processors? I doubt though.