Hi everyone,
A couple of days ago I updated two Xs from the latest 14.x to 15.1. System animations on my Xs are sluggish (similar to a frames drop effect). However, my wife's Xs is like before, fast and smooth. I have been trying to figure out what is the difference between them but in vain. We bought an iPhone Xs 64Gb together in the same US Apple store in December 2018, have an active backup to iCloud, ±20Gb free space, almost identical system settings and apps, similar battery health (81% (mine), and 86%). I guess this is a software problem and hope it will be fixed soon.
By the way, I bought Pixel 3 in November 2018, some months ago I updated it to the latest Android 12, it works as fast as on the first day, with no problems at all. iPhone Xs and Google Pixel 3 were released in Autumn 2018, they are in the same category in sense of performance. Therefore it would be a shame for Apple not to fix this annoying bug.