Analyze low performance scheduling

The scheduler takes some time to schedule new origins.

It's now more apparent since we added some more workers to consume the standard git queue [1]. It was a bit apparent with the gitlab.com origins (still lagging but less than earlier [2])

• [1] #3825 (closed): the queue is now drained completely but the scheduler takes too much time to fill it.

• [2] still 409k origins lag (vs 3M at first) with an apparent plateau around the 01/01/2022 - https://grafana.softwareheritage.org/goto/hNSnn-Ank?orgId=1

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Migrated from T3827 (view on Phabricator)