I think my problem is solved!

After everybody's help narrowing my problems down to the phone and not the software, I looked on some Android forums and found these ideas to speed up Android, particularly for the S3

1) Boot to recovery mode and clear the cache
2) Go to settings-> developer options-> turn off window, transition and animator scale

After making these changes Primo is working much better. Speed calculation is faster and more accurate, and the map display position is good up to 100 km/hr

Thanks again to everybody who contributed to this solution