I think the idea is useful for large groups however a simple "cool down period" is far too simple. Something like "10 messages an hour" would be far more useful

I think the idea is useful for large groups however a simple "cool down period" is far too simple. Something like "10 messages an hour" would be far more useful I think as you can still have quick and efficient exchanges while preventing the ability for any single user to be dominant. However even that is too simple. You would need to pair it with good UX so users aren't stuck without messages when someone is asking them a question.