Ubuntu disk IO slow for a while, and suddenly fast again...?

This is on a laptop, a Thinkpad X250. I'm running Kubuntu, but I don't think this is relevant.

For several months now, I've been feeling like my disk speeds were poor, especially considering that I have an SSD. I don't remember exactly when it happened, since most of the time my day-to-day activities aren't affected severely. But boot is much too slow, and benchmarks I did at one point (no longer have them) seemed off.

I upgraded to 19.10 a couple days ago, and didn't see any improvement. If anything, IO behavior got worse, with UI freezes when copying large files (reasonably high write speed though), something that I hadn't experienced before. But that's a story for another time.

Anyway, I was doing some other maintenance yesterday. I have encrypted LVM, and the changes I made were as follows:

- changing the volume and filesystem names of the main install (3 guesses why) - remove old, unused installations - copy my boot partition from #5 to #3, keeping UUID in tact

Broke fstab due to the logical volume name changes, fixed it, updated grub, and everything seemed fine. Also did disk benchmarks while running the live CD, just read speeds, and those at least seemed fine.

My computer died overnight due to unrelated stupidity regarding power management. So, I charged and rebooted this morning, and suddenly my boot process is way faster, and other things feel snappier as well.

So my question is, what on earth could have fixed things? Again, if anything 19.10 and the new kernel made things **worse** immediately after upgrade. Off hand, I didn't notice this immediately after fixing grub, though even assuming that's what did it, it's still a mystery why. I don't think I've upgraded the again since the release upgrade either. I've spent hours Googling these kinds of issues on several occasions, and never seem to make any headway. Any ideas?