You'll get much more accurate data on the track with an external GPS. At high speeds, a phone's built-in GPS is not enough. Very easy to add an external:
XGPS150 is 5Hz (5 times per second) and $82:
http://amzn.to/2pncia7
XGPS160 is 10Hz (10 times per second) and $135:
http://amzn.to/2q7JCzB
I use the cheaper XGPS150 and it works well.