Because changing lock screen styles is very resource intensive and the editor in One UI 8.5 is not well optimized.
It isn't like iOS or Pixel where you are using static presets. On One UI, every asset is movable/resizable and the preview must be rendered in real time.
The optimization issue comes from the fact that the lock screen has basically been "under construction" since at least one UI 7, with constant additions every release like new fonts, new font weights, adaptive clocks, real time color mapping, and real time blur on clock digits.
If you're using LockStar, you may be able to speed things up a bit by uninstalling it (removing some of the extra weight)
