CI is too slow

CI used to take ~5 mins total, but is now commonly taking over 30 mins. This is extremely limiting, especially now we have enabled merge queue. Items often spend an hour or more in the queue, and can then drop out due to flakiness.

Investigate which CI jobs are the slowest and why, and find fixes.

Start by reviewing recent CI runs and comparing the durations of each job. Investigate the slowest jobs and the reported flakiness, then verify that proposed fixes reduce total CI time and prevent items from dropping out of the merge queue.

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