Following similar posts on poor quality GPS data, I did some of own checks on a simply road loop, I found the data was reasonable accurate at around 35km/h, at a recent ride doing hill loop on the decent at 12.8% I was reaching 55km/h or 35.5 mile/h, what I have observed the higher speed causes the GPS to record to a straighter line mean its predicting the next GPS location and auto-correcting as it re-aligns , the picture shows lines(different colors represent speed groups) the blacks lines are 15+ metres per second(54km/h) and you can clearly see they show a very straight line, I am slowing down at the apex due to the off camber, noting that the slower decents are more inline with the road. what I have also observed on other people rides on MTB where they take sharp corners at higher speeds due to ruts in the tracks etc is they are going off segment due to the predictive GPS points being generated similar to a centrifugal force.
The faster you go the straighter your GPS track will be and it will re-align back to as you progress, or the slower you go the more accurate GPS path you will have.