It's the RAM. It needs to "trained" which takes some time but for for some reason these boards seem to randomly forget their training, requiring it to happen again.
Doesn’t affect soldered memory or lower speed memory (like DDR4). Many memory controllers fail to achieve good speeds and timings at all on 4 DDR5 DIMMs, and fall back to running DDR5 at 3600MHz instead.
That being said, it's not hard to get a hold of a reasonably modern DDR5 EPYC board. Something like this: https://www.phoronix.com/review/gigabyte-mz33-ar1
Expensive though.
I distinctly recall thinking my LPDDR5 NUCs were broken since they seemingly didn't boot the first time, until I recalled the training stuff. Took up to 15 minute on one of them. But neither has had any issues since, hence my question.
Memory training has always been a thing: during boot, your PC runs tests to work out what slight changes between signals and stuff it needs to adapt to the specific requirements of your particular hardware. With DDR4 and earlier, that was really fast because the timings were so relatively loose. With DDR5, it can be really slow because the timings are so tight.
That's my best understanding of it at least.