This morning I installed the update on my Pixel 9a. All done, I let the phone reboot and waited for apps optimization to complete, all 116 of them (also waited for the phone to cool down, it seems that the optimization process is very CPU-intensive).
After a while, it seemed like the phone became... sluggish, switching apps took longer than expected, navigating menus in apps also less responsive (I even happened to wait for around 3 seconds for my favorite contacts to load in the default dialer app). I don't think I've changed anything of note to the apps running on the device or how I've been using it since.
Tried the old mantra of "turning it off and on again" and, upon reboot, the system showed another notification saying it is optimizing another 11 apps; I was a bit confused, but I took it as a good sign, maybe some key apps were not optimized properly and that's the culprit of the degradation.
However, after that process ended, it didn't seem to improve at all. Moreover, the optimization process did not conclude with the same "Restart apps after optimization" notification that I was used to clicking after a finished system update, so maybe they were not optimized after all? Maybe the process breaks down somewhere?
I'll try turning off Developer Options, but I don't really want to also disable Accessibility, it would impede much of my muscle memory of using a phone to do so.