iPhone 11 keeps freezing and lagging

Try the following, in order. Stop at the particular step which resolves the issue.

• Restart your iPhone - Apple Support
• Force restart iPhone - Apple Support
• Reset your iPhone's Wi-Fi settings: Settings > General > Reset > Reset Network Settings
  • This resets Wi-Fi networks and passwords, cellular settings, trust settings, and VPN and APN settings that you’ve used before.
• Perform a "Reset All Settings" reset: Settings > General > Reset > Reset All Settings
• Reset your iPhone to its factory default settings: Settings > General > Reset > Erase All Content and Settings (Note: This reset will erase all of your iPhone's data and media and resets all of the its settings. You will need to restore it from a previous backup from either iTunes or iCloud.)
• Make a Genius Bar appointment at your local Apple Store or take it to an Apple Authorized Service Provider.