[bmhin]

It likely depends on the broad culture behind it. If someone sends a "Hey" and then is a fast enough typer and immediately can get the context going as I look, that's not a huge deal. Sometimes it's an opening to a more synchronous conversation and then it's less of a big deal because we're gonna be going back and forth with quick messages. Part of the irritation is you can still just always open a message with "Hey, something something something" and keep it all in one.

The problem is people who send a lone "Hey" and then... nothing.

Some will wait for you to say respond with your own greeting (which is all you can respond with because you don't know what you're even talking about yet). Other's don't wait but do delay and the initial greeting was just a warning or something that a "real" message is incoming. I've had folks ping me the "Hey" and then I respond immediately with a "Hey" (to stave off the above "permission to speak" people) and they get distracted and say nothing else for 10 minutes.

Some will message you while you're busy, so you don't respond to their "Hi". You then respond to it a half hour later with the only thing you can say to that contextless message: "Hey, what's up?". Only now maybe they are busy, and they respond again to you a half hour after that. You've had a conversation "opening" for an hour for what purpose?

Some will immediately begin typing out a gigantic block of text and you are sitting there waiting while the "XX is typing" message unwaveringly sits at the bottom. Bonus points here if they were editing and revising as they went and you get an eight word sentence after what appeared to be 3 minutes of continuous typing.

Some will do all of those things so you end up having your attention taken multiple times for minutes at a time for a message that realistically might take you a couple seconds to read, parse, and respond to if it had been sent all at once.