Explanation: My Best Performance is based on the fitness value you achieved. The theory behind that is your most economic run — not necessarily your fastest.
On Fitness: In TrailRunner, the fitness is a calculated value based on the average pace and heart rate of a workout. The idea is that if you where very fast with a low heart rate you are fitter, while the opposite applies if you where very slow and had a high heart rate. This value is just for orientation and does not have the accuracy of the methods used in Polar watches or other biometric methods, but it gives a pretty good overall impression. To compare your fitness values to average adults, take a look into this table.
Reason: TrailRunner loads maps from internet services that provide free maps. If the loading process is slow, the map service is slow and TrailRunner can't get anything faster. Another reason are "fair use" agreements for commercial web services. This means that as long as you get maps in a fair amount and frequency, the commercial service won't put you on a black-list. For that reason TrailRunner throttles the keyhole imagery service.
Solution: TrailRunner caches maps. If you have once loaded map fragments from a service, it's unlikely you will need to load the same fragments again. The longer you use TrailRunner, the faster it will become.
Tip: In case the map material on the web service was updated, as it is frequently the case with openstreetmap.org, you can use the File > Consolidate > Reload background maps command to reload what’s currently visible or you can remove the MapViewTiles folder in your Application Support/TrailRunner folder.