A slow pipeline is an observation, not a diagnosis. Downloading dependencies, unpacking packages, compiling code and running tests can all appear in the same build step. Changing a runner or adding a mirror before separating these activities makes it difficult to tell which intervention helped. Begin with the question the team needs to answer: is dependency retrieval the constraint on this workload, and what evidence would justify changing it?
Build a comparable baseline. Select a representative repository and record the commit, dependency lockfile, package-manager version, runner resources and configuration. Split fetch, install, compile and test timings where the tool permits it. Note retries, upstream errors and authentication failures. A run that succeeds only after several retries should not be presented as a clean success simply because its final stage is short. Avoid writing credentials or private registry URLs containing tokens to logs.
Keep cache state explicit. A clean run and a warm run answer different questions. Clean runs can expose acquisition cost; warm runs show behavior when reusable data exists. Neither is inherently the correct headline number. Repeat each relevant condition, record the sample count and retain failed observations. A branch switch, changed runner image or different dependency version can invalidate a direct comparison even when the project name is unchanged.
Test one explanation at a time. If retrieval accounts for little elapsed time, investigate compilation, test design or runner capacity instead. If retrieval is material, review the approved upstream and the actual network path before piloting a new registry or mirror. Do not conclude that a location causes a bottleneck merely because the runner is there. Measure the environment you intend to change; another organization's reported outcome is not your baseline.
Protect correctness during the pilot. Keep package identity, lockfiles and integrity checks constant. Record any configuration difference that changes the inputs. A faster build that accepts an unverified artifact or silently falls back to a different dependency is not a successful optimization. Check failure behavior as well as healthy timing: the team needs to know what happens when the chosen dependency path is unavailable.