- For shitty posters, it does not result in any changes in post quality. They simply post their first post, then play games, and 30 minutes later come back to show their shit again, repeatedly till they hit post quota.
It would slow things down at least a bit
I'm not convinced it would. We already know that many spammers save a bunch of pre-written posts as drafts or in notepad, and then just burst paste and post them as required to hit their posting target for that day/week/whatever. If would be fairly easy to write a script to do this for you (I suspect many already use such a script), and then just add in a 30 minute delay between posts. And if that's too complicated, then they don't even have to do that - they can just write each reply in a new tab, and then they only have to click "post" every 30 minutes. A script to do that is completely trivial.
Spammers find a way.