What replaces this natural unconscious perception, is an effortful, hyper-focused search for cues. A malfunctioning sentinel scanning, searching, scouring! This type of processing is tiring.
Throughout the course of 5 hours of meetings today, the least tiring was an hour long coaching phone call. I prefer to use the phone because it is just one channel of information and attention, and I know I am not going to search for other cues.
You will know the unusual tiredness too if you have had 4-5 hours of video calls. This National Geographic article helped me understand that better.