While listening to non-natives can be tiring, spending the day speaking in a foreign language is exhausting. I’m not sure if they explained the reason for this phenomena in your training. When we speak with those whose language we have in common, we can give our brain a rest and cut in and out of the conversation (even momentarily) and still follow the thread (as we tend to fill in the gaps of information we weren’t really paying attention to). For example, you can probably finish the following correctly: Well, you know, when in…Romans do. Not native speakers can’t do that, nor do they necessarily have predictable language patterns. As a result, we must always be “on” when communicating with non-native speakers (actually, even heavily accented native speakers…I find I am exhausted after spending time with someone in the deep south or England).

Just a note, your insights (multiple channels of communication to please everyone) was also one of the conclusions my students often come up with.