A lot of jobs are "almost queued" even though identical ones already exist, waiting to interrogate github about which commit to build at all.

GitHub API Usage: The quota may have been refreshed earlier than expected, rechecking... 08:52:17 

GitHub API Usage: Current quota has 1334 remaining (2151 over budget). Next quota of 5000 in 51 min. Sleeping for 34 min. 

New queue items appear every scan period (e.g. 2-5 minutes), see screenshot.

And why does a checkout go through api rather than directly git?
API makes sense to discover repos and PRs, but not to check out?..