If you are experiencing OneUI 7 bring slow, do this:
Connect your Phone ti a PC or laptop. This is needed as Samsung doesn't want unauthorised access to the Recovery Environment.
Hold Power (aka Side button) and Volume Up button.
You should see the Samsung logo pop up. When you do, release the power button, but KEEP HOLDING THE VILUME BUTTON.
When you see a menu for "Android Recivery", use the Volume Down button to scroll down to "Wipe Cache Partition". Select yes, and let it do its thing.
When its finished, use the Volume down button to scroll down to "Reboot Device".
It should be better after that!
For more info, go here: