Restart iPhone
Clear Safari cache
Settings → Safari → Clear History and Website Data
Settings → Safari → Advanced → Website Data → Remove All Website Data
Clear app cache (for specific apps)
Open the app → Settings/Preferences → Cache/Clear Cache (if available)
Offload unused apps
Settings → General → iPhone Storage → Offload Unused Apps
Delete and reinstall an app
Settings → General → iPhone Storage → select app → Delete App
Reinstall from App Store
Clear system data by resetting (last resort)
Settings → General → Transfer or Reset iPhone → Reset → Reset All Settings
Reset network settings (can help with cached connections)
Settings → General → Transfer or Reset iPhone → Reset → Reset Network Settings