Since you are storing your files in a Google bucket, are you using Reference Artifacts? This should change download to be from a bucket itself instead from our servers. The bucket download should be quite a bit faster than doing it through our servers.
After looking around, the difference may be due to the recursive download that you are using for GCP. We are using defaults for the artifact download so it looks like the difference is purely based on the API calls but I am investigating further into the differences. However, as of right now, there isn’t a faster way to download artifacts since we download reference artifacts with only one method.