Investigate how to reduce the duration of the `gitlab:assets:compile pull-cache` job to under 15 minutes

As can be seen in https://app.periscopedata.com/app/gitlab/496118/Engineering-Productivity-Sandbox?widget=6914312&udv=785399, the gitlab:assets:compile pull-cache job is now taking more than 21 minutes:

We should improve it to finish under 15 minutes.