I have found that my phone starts to feel real slow I do hard resets. If that doesn’t work then every few months I will plug my phone in my computer. I will put in dfu mode. Then I will restore and update. This will reinstall the whole OS and erase everything (back up data to the cloud). Then I will setup my phone as new and not restore from backup.

Another option would be to get a new battery as the device is anywhere between 3 and 5 years old and batteries degrade. Old batteries will slow performance down.

Service provider maps are also fairly misleading. Depending on what carrier you have service may vary. I’ve had times where one bar has been a very good rock solid connection and 5 bars be completely useless.

Lastly perhaps a newer phone. An iPhone 12 or newer. Apple replaced the intel modems with Qualcomm modems and they are much better. This ignores 5G. Sometimes it’s better other times not. I am on AT&T and 5G is less than half the speed of LTE(5GE). The only way to get access to all the bands on AT&T is to have an iPhone 14 series or S23 series.