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.
Make a bounded decision. Compare repeated observations and identify the conditions under which the change helped or did not help. Assign an owner for the configuration, monitoring and rollback instructions. State remaining uncertainty rather than promising a universal percentage improvement. A sensible result can be to reject the mirror proposal and fix a different stage.
For help turning measurements into an agreed pilot, see DevOps implementation services for teams in China. This is an implementation discussion, not evidence of a guaranteed performance gain or prior local deployment.