It's a large update being a new Android software update and Samsung One Ui 3.0 update so may take a little while indexing i.e files settling in.
The update itself will have the phone “working hard” to get it set up.
If it continues after short while then what I suggest is to clear the phone's system files cache.
Clear your phone's System Cache Files by turning your phone Off and then from a set series of button presses go into the phone's System Files and clear the cache. No information is lost this way.
A last resort is to back up and remove any relevant sd card and factory reset.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.