Gitlab also has some tips here: https://docs.gitlab.com/ci/caching/ on using shared caches, which can help in some scenarios, especially runners in Kubernetes that are ephemeral, ie. created just before a job starts and destroyed afterward.

tldr; "A cache is one or more files a job downloads and saves. Subsequent jobs that use the same cache don’t have to download the files again, so they execute more quickly."

It will probably still be slower than a dedicated runner, but possibly require less maintenance ("pet" runner vs "cattle" runner).