This has been my bugbear for years. Even in places with good cell service on average there's a hundred individual places that have terrible service. Also a good signal to the handset doesn't necessarily mean good actual service. It doesn't even require traveling, just normal daily movements to get wildly variable network performance.

It's infuriating when it's obvious that the developer of an app only ever tested it in a simulator on their dev machine on their super fast WiFi. It never seems to connect with those people developing a mobile app (or web app) that the "mobile" part has a meaning more than just on a handheld device.