Actually, I’ve figured it out. Something has changed about thread scheduling. Maybe intentionally, maybe not, but it’s definitely different.
I set my background threads to the lowest priority constant since they aren’t user-facing. In 2025r2.1, setting those threads to normal priority helps. The read-intensive operation returns to within margin-of-error of 2023r4, but my write-intensive operation is still slower at 12 minutes 34 seconds. It’s sure an improvement over 21 minutes, but a far cry from 53 seconds. My guess is high priority will get the rest of the performance back, but I haven’t tested that yet. It’s clear the thread priority plays a major factor here.
So now the question is what changed? Going to comb through release notes again. Now that I know it’s thread-related and not database-related, I might find something relevant.