Just to offer a different sound, I recognise none of these issues. I’m pretty content with the update, I have no high prio issues. My task switcher occasionally breaks (it fully reloads every app then) but a reboot fixes it. I have 0 performance or memory issues aside from that. It runs just as smoothly as A14. And I also don’t know if A15 differs much from its predecessor in terms of performance requirements.
I’m just trying to say, I think your device has specific issues and I would contact FP about it directly if you haven’t already.