Hey, there can be multiple things which may cause this issue, try the below troubleshooting:
1. Check if there isn't any third party app which might have caused the device to slow down
2. Your device should have a few % of storage left as less free space on the device can cause a device to run slow
3. Ram management - there should always be adequate RAM available if the device needs to work flawlessly
4. Restart whenever you face this issue
5. Restart the device in safe mode – i. Power button hold for 2 seconds, ii. tap on the power off message which pops up on the screen, hold for few seconds it will give you an option to boot in safe mode.
6. Also, check if your device is on the latest software version, if not already updated
7. If nothing work you can back up your phone & factory reset your device: :
https://www.asus.com/support/FAQ/1006346/8. Last option would be to visit the service center to have
a check. Locate here:
https://www.asus.com/in/support/Service-Center/India