My phone is always in power save mode. Re-enabling transparency actually made the UI less jerky. It was mostly the keyboard that became unresponsive, I could type 15-20 letters while it froze and it would then „catch up“.

Also reported here for example: https://discussions.apple.com/thread/255911835

Re-enabling transparency improved this a lot, also keyboard still hangs a bit from time to time. I’m always in power save mode, on an iPhone 12, running iOS18.