Everytime I train my Fenix 6 series pace is off by at least 3-4 seconds compared to everyone else I’m running with even when I’m running directly with them it’s been happening for the last 6 months.
Everytime I train my Fenix 6 series pace is off by at least 3-4 seconds compared to everyone else I’m running with even when I’m running directly with them it’s been happening for the last 6 months.
Just run faster 
Now seriously - the watch uses the combination of GPS data, and the data from the accelerometer to estimate your pace, so if you get slower pace and shorter distance than others, then the calibration of the accelerometer data may be off. Not necessarily on your side - theoretically it may be a problem of the other runners (although less probable especially if they are numerous with identical data).
If you use the HRM-Pro strap, the accuracy is much higher, and the calibration can be reset anytime. Unfortunately, if you do not use the strap, the only way to reset the calibration and do it anew, is by doing the master reset of the watch to factory defaults. Or just continue running in open space under good GPS conditions, and the continuing calibration may eventually finally adjust to a more accurate level.
Frankly told, personally I would just get the HRM-Pro, if it is within your budget. Its impact on the metrics (not only the pace & distance, but also HR, LTHR, HRV, Running Dynamics, etc) is such a great update, that is well worth of the investment.