Timer is slight off by several seconds with app and treadmill (or app and stopwatch, or any other timer)

The reason for the issue you’re describing is when the device screen is turned off (or put into sleep mode), the OS will slow down the processor, so as to not drain the battery. This slow down and shutting off of certain features by the OS could also slow down the timer counter for our app. Every phone manufacturer has a different OS overlay on top of the standard Android OS, but our dev team is continuing to work to improve the app performance for this issue.