The common theme for us seems to be that we expend a lot of energy when talking with others. We analyze what they say, in order to come up with thought-through responses or follow-up questions. We don't do "chit chat" as such.
Before such events I dread them, because I know it will feel exhausting. Yet when I'm at them I have a good time. Afterwards however it feels almost exactly like I've been programming hard for a whole day, my brain is spent.
I, unlike my SO, also has some social anxiety. I'm very likely somewhere on the spectrum, as I often feel difficulty getting social interactions right. So I spend some extra energy thinking about what the right response is to this or that, or how to interpret body language. Especially if it's a new setting or a lot of new people this can cause a lot of extra mental work.
This part however has gotten better with the years, mostly because I've stopped caring so much what others think and just let myself be myself more.