Profile each stage first. Often one or two steps account for 80% of the time. Quick wins include dependency caching, test parallelisation, and splitting the pipeline into fast feedback and thorough verification stages.
Strong answers describe a systematic approach: profiling each stage, identifying the slowest steps, parallelising independent stages, caching dependencies, using incremental builds, optimising test suites (running fast tests first, slow tests asynchronously), and right-sizing build agents. Best candidates discuss the trade-off between pipeline speed and thoroughness.
Practical DevOps skill. Slow pipelines reduce developer productivity and discourage small, frequent commits. Candidates who have systematically optimised a pipeline demonstrate operational awareness.