Some people treat email and chat like sync communication, getting annoyed if there isn't a response within a short time. Ever have someone walk up or use chat to ask "did you get my email (that I just sent 5 minutes ago)?"
Async is great for initiating conversations or going back and forth a couple times. Go much more than that, and sync often (though not always) becomes easier and faster.
An example of an ideal flow to me:
Person A (12:34): hey, I'm trying to implement this new API call, but the parameters we came up with kind of conflict making it more complex than we thought
Person B (12:52): Hmm, maybe there's a way we can simplify those. Have time within the next hour to meet/conf/video chat on it?
A (12:55): just about to grab lunch, how about 1:30?
B (12:56): ok
A (1:26): alright, back, ready anytime
B (1:29): (initiates video chat)
(At end of call): ok, let's run this past rest the of the team and give us a day or so to think about it, and see if anyone has objections or we think of other implications. I'll send an email summarizing.