Cable not charging fast

Hi, try to change the cable or use a different charging brick.
If that doesn't work try to charge it in Safe Mode, Safe Mode puts your phone in a diagnostic state (returned to default settings) so you can determine if a third-party app is causing your device to freeze, reset or run slow, or slow charging.
1-Press and hold the Power button and Volume down button (right edge) until the 'Power off' prompt appears on the screen then release.
2-Touch and hold Power off until the 'Safe mode' prompt appears then release.To confirm, tap Safe mode .
The process may take up to 30 seconds to complete.
3-Upon reboot, 'Safe mode' appears in the lower-left of the unlock / Home screen.With Safe Mode enabled, ensure Airplane Mode is turned off then test device and app functionality, then delete the suspicious apps.

If all the above doesn't help please visit the nearest Samsung service centre for deeper diagnosis.

Please let me know if this was helpful.

Try with other charger may be ur charger or cable damage

Hi,

Please bring your device to a Samsung Service Center and have it checked my a technician to help resolve the issue.

Find the nearest service center to you online, or on Samsung.com 

Hi there, unlike the old mobile phones, the newer one which comes with fast charging will sent some signals to the charger once connected. If there is any issue with the data cable, the signal which was sent to the charger won't reach the charger and the charger won't supply enough voltage and current. This will result in slow charging. You can try with another USB cable.

Just try with another cable. The cable might be damaged. When you bend it or fold it beyond a limit, the wires inside may get streched and loose it's conductivity. And your intelligent S21 may just detected it. Inform us your results. Thank you.