who can make 15-25 posts/hour?
Quite honestly , I could make 15 posts per hour.
I would need around 1 minute to read the OP and ignore the rest of the thread and I could easily formulate a reply in another 1-2 minutes , leading to 20-30 posts per hour.
But the fact is that if you spend time reading each and every reply , there is a great chance that someone has already asked what you have asked or brought up the point you were going to. You then end up posting once every few threads that you read , probably once every 3-6.
On the other hand , a person who owns a Marketplace thread , maybe a service , a goods sale thread or a gambling thread would definitely be expected to have over 15 posts per hour. They would need to answer each and every question by a potential client , regardless of whether it has been covered before or whether it is silly and doesn't really warrant a post.
TLDR ; 15-25 posts per hour can be non-spammy, but only in rare cases. The first of which I could think of was owner of a sales / service thread.