A couple of tips I picked up when setting up the finger prints are.
When setting up take x3 scans with each thumb and then rotate the phone around when taking further prints of each thumb so your phone can read both thumbs from the same finger print scan. Scan your prints as you would normally hold the phone.
Also using two fingers when setting up one fingerprint works pretty well.
Go into Settings . Biometrics and Security. Biometrics Preferences . Turn Off Screen Transition Effect.
Then go into Settings and search for Optomise Battery Usage. Tap in Optimise Battey Usage tap to show All and scroll down to com.samsung.android.biom and toggle Off
For me this improved the fps on my Note 10+ 5G which I appreciate is a different model to yours but should be the same outcome- hopefully.
🌈 Stay Safe & Stay Alert. 🌈
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
Don't upgrade to the S21ultra if you live outside of the USA because the Exynos 2100 chip has the same old rubbish fingerprint scanner as my note 10+
IT IS TERRIBLE!!! See the YouTube video I made to show it took me about 100 tries to add my left thumb, then it failed. VERY BAD Samsung!!! https://youtu.be/4Mp8ECNUpgQ
Certainly the Exynos version is very bad... same as the Note 10+ I have. This is DEFINITELY NOT an ultra-sonic fingerprint scanner in the Exynos. Maybe the Qualcomm version is great, but this is extremely disappointing and is the reason I returned my Galaxy S20, Galaxy Note 20 Ultra upgrades after a few days, and now it looks like I will be returning this phone. It is not a substantive upgrade on my Note10+, the same annoyances.