If the freezes and lags are due to minor firmware problem or glitch, then this procedure is more than enough to fix them.
Press and hold the Volume Down button and don’t release it yet. While holding down the volume button, press and hold the Power key as well. Keep both keys held down together for 15 seconds or until the Galaxy A20 logo shows on the screen.
When the phone has finished the reboot, try to see if it still keeps freezing and lagging. If it does, then let us know.