Hi John, it is highly recommended that you wipe the cache partition after performing software update as this can solve lag and stutter and minor issues. I'd suggest if you wipe the cache partition on your phone as this should fix the issue. https://www.sammobile.com/samsung/galaxy-s21/wipe-cache-partition/
Same here, I installed the update yesterday and since then my phone has been freezing and restarting on its own. If you resolved the issue please tell me how