Try a speed test to make sure it is not your mobile phone's data connection that's the issue. It's easy to use and you will quickly get a message letting you know if your connection is fast enough to play.
Speed Test for Pokemon Go is not associated with the official Pokemon Go game, Pokemon brand, Niantic or Nintendo.
Please keep in mind since this is a speed test app it will use a lot of data. Keep an eye on data usage to make sure you're not going over your cap.