I guess it would depend on the strict definition of "constructive".  If that doesn't necessarily include post length, 10 posts in an hour is not only possible but I think I've done it before and probably others have as well.  Sometimes there are discussions where there's actually back-and-forth conversations between members (which unfortunately isn't the case most of the time), and if that back-and-forth is fast enough, it'd be easy to make 10 or more in an hour.

I don't think rest has anything to do with posting.  It's more a matter of finding threads that catch your interest and in which you can write something halfway decent instead of just writing a very generic, vague, and poorly-constructed series of words--and that's generally what burst posters do.  I've seen them in action plenty of times, and I can say with certainty that there's only one exception that I've ever seen to that rule but I'll be damned if I can remember that member's name.  He got called out for it, but the stuff he was writing was by no means crappy.