Why is the current pace always inaccurate during a run on my fenix 6 pro sapphire?

Especially during the first km where it always seems to be recalibrating, but then even further during the run it may show a pace of 6:00/km when in reality it may be 5:30/km or more or less. It’s quite disappointing and unsettling as I cannot rely on the data and train accordingly.