The default is 15 seconds for a new topic, and 5 seconds for a reply/post. Topics and PMs are treated in the same way, and typically creation of a new topic or PM will take more than 15 seconds. In six years and across a broad number of communities I’ve never received a complaint from a user over hitting this limit, have you raised your defaults?
Sounds like these users are rapidly copy pasting the same content and sending it to other users? They would have to be doing it quite rapidly to hit the rate limits.
Yes I have no problem changing the setting. I didn’t (knowingly) change it, although we have been on Discourse for nearly 5 years, so I may have set it and forgotten. The other possibility is that the default might have been different way back when? We were on Discourse when it was still in beta.