I am under impression that upon upload from external devices Strava discards any speed data and then rebuilds from distance distance changes over time. As a result the running pace graph looks much more noisy and spiky than pace graph for the same activity in e.g. Garmin Connect. I'd say that in general it is very hard to use Strava pace graph for any kind of analysis - it is just too noisy to see any trends. Perhaps Strava should either be applying some smoothing or, when available, just use the original speed/pace data from the device. Many modern devices fuse the running pace data from both GPS and accelerometer which makes it way more stable than GPS based pace alone, and Strava should try to preserve that.
