Pokemon-go – Is total distance walked specific to a single Pokémon, or to a species of Pokémon

Anytime the "loading" symbol appears in the corner, your distance is usually not being counted. If you wait for the game to finish loading before you walk, your distance will almost always count. The update is dependent on whether it is able to get a good GPS signal but even if you have excellent signal, if the game is loading it will not update until it has finished loading. At least, according to a number of experiments done by a few people that seem to show that distance is not counted while the game is loading. This would explain the jumps you experience as well, I would think. Hope this helps.

On August 1st you would have been experiencing slowness from an API change Niantic made to begin preventing access from external sites.

There was a significant delay for any communication involving GPS, which includes Pokemon appearing in your field of view.

After Niantic fully blocked everything a couple days ago, you should be seeing significantly improved performance....although a severe lack of any means to track the Pokemon nearby :(