Just FYI, I also ran similar test measuring execution time of runOnce API and got similar results. Typical execution time is less than 1 second, but there are frequent spikes (several times per hour) when execution time exceed 10 and even 20 seconds.
I got 6 occurrences exceeding 20 seconds within the last 4 hours:
16:40 PM : 43.4 seconds
16:41 PM : 21.4 seconds
16:46 PM : 20.6 seconds
17:35 PM : 20.1 seconds
17:41 PM : 37.9 seconds
17:42 PM : 37.3 seconds