64K is generally small recordsize. For media they recommend 1M. My db was sitting on the default 128K recordsize and perforned fine on 10.10. It doesn't matter much for reads because ZFS keeps it in RAM and if it gets evicted it goes into SSD cache (l2arc). But for writes the cache wouldn't help unless I disable sync which risks data loss. I don't think 10.10 did much writes during lib scan so write perf only mattered when something changed.

With great power come great .. uuh .. :D

Also the meme is just bait so you read abt the problem and see if you need to solve it for your setup.

Not sure if this is the problem I was hitting with my Home Videos library type. It's got directories with YouTube videos. I tried re-adding it in 12-rc3, which was released a month after the nightly mentioned in the issue thread. Still couldn't complete. I ended up re-adding the media as Shows library. Works okay with the directory structure I have.

The write performance was recorded without the problematic library (deleted).

That would be reasonable. I did repeated rescans and only counted subsequent rescans. For me the initial scan after upgrade took a bit more but not hours. Subsequent scans took less. E.g. 20min -> 13min for write-optimized filesystem. So that's reasonable, although 10.10 was way faster. Library scans are expected to get faster in 13 according to some Github threads I read.

When I had the broken Home Videos library I waited 3 days for the initial scan to complete and it did not. Repeated rescans did not seem to complete although I didn't wait 3 days for them. I'm not taking into account those scan times. Something was wrong with this library type on 10.11 and/or my media. Worked fine on 10.10.