If you carry cell phone on course, that is really not safe (e.g. fall off and have the phone puncture your body, shatter glass in your skin, and other possible issues).
Also, if you need an app to track speed while on course, regardless of type, you have a lot bigger issues and really need to go back to the beginning to learn to ride.
I can understand using an app with the phone mounted to your arm on a gallop to learn speed in training.