Apparently, thats the way it is!
Been dealing with this erratic pace since i got my Fenix 5 Sapphire in March 20. On May 1, I started communications with support regarding this very irritating issue and after more than 15 emails back and forth on which they confirmed the erratic behavior they are now telling me, after 5 weeks of trying every combination of sensors, settings, and firmwares without success, that this is the behavior i can expect from a $700 gps watch. Nice one!
So, if you guys have any hope of getting your pace performance corrected, forget it! Per support feedback, this is standard GPS behavior. Nevermind my previous 735XT performed flawlessly in the same routes i now deal with the F5.
Bologna!