Hello,
Whenever you're facing connectivity issues, always questions both ends that may cause it. First of it being the carrier and signal strength, and on the other end your iPhone.
Cellular is depending on the signal strength carried out by your carrier. This means that depending on your location, it will be slower or faster depending on the strength of the signal.
However, if you're having this issue anywhere. Where you usually had better signal and faster cellular it's time to do some troubleshooting.
- Force restart your device (Hold Power + Home Button untill you see the Apple logo on the initial startup screen)
- Update Carrier settings (should automatically prompt when connecting your iPhone to iTunes, or whenever you go to Cellular settings)
- Clean your SIM-card
- Reset Network Settings
- Restore your iPhone to factory settings (create a backup first!)
- Contact carrier about your issue
Always keep in mind, before starting to troubleshoot. Test it in all situations where you'd normally have a good and fast connection.
Kind regards,
K