See parent for more background, but summary of the issue is that concurrency limited builds don't execute retry tasks on a tight schedule (default is 300s). Instead, the gap between retries is anywhere from 5m, 30m, and I've seen 5 hour gaps.

Build task queue contention might be able to explain away a 10 min or maybe 30 min gap, but not consistent gaps and certainly not a 5 hour gap. To me, this feels like something else is happening here.