When Auto TWA is enabled, your boat adjusts its course automatically in order to keep the same wind angle at all times. These course changes do not require any tacking or jibes, and therefore do not slow your boat down.
Auto sails allow your boat to choose the best sail automatically based on existing weather conditions. This being said, sails still need to be switched, and this does not occur in the blink of a eye. Your boat will slow down for a bit when you switch sails, whether you do it automatically or manually.