@AnnyMallow: I'm sorry to hear that you've been experiencing performance issues with your Galaxy S21 Ultra. You may have attempted these steps already, but please try pressing the Volume Down and Side Keys together for around 10 seconds until the phone restarts. Don't worry, this won't affect your personal data or settings. Now, head to Settings > Battery and Device Care > Optimise Now, followed by Memory > Clean Now. You should also head to the Google Play Store and Galaxy Store, and make sure your apps and features are all up to date.
If the issue persists, I recommend reaching out to our Help Desk via the link below, and an advisor will be able to run through further troubleshooting steps with you.