Activating [A Gentle Touch] while also using his Spiritual Sense and going around digging, Di Tou's face paled another shade, and when he started working, he was noticeably a beat slower. Although [A Gentle Touch] didn't have any 'costs' in the sense of using his Qi or Spiritual Sense, it seemed like it wasn't entirely free, as it still put a slight mental strain on Di Tou when he used it.
As for why this was the case, it should be said that in part it was due to the fact that Di Tou wasn't entirely proficient in the technique, and thus had to think and focus separately to use it.
Imagine counting to ten thousand while gardening, or playing a video game. While counting isn't necessarily hard, just trying to add another action to multi task on wasn't exactly pleasant. Much less Di Tou who was trying to do three or four things at once.