Originally Posted by CbadRider

You can go through all of your posts and delete them one at a time if you want. Those with less than 100 posts might do it and it won't have much of an effect on the server. People with thousands of posts aren't likely to do it, and those are the ones that will cause hiccups.

Big, popular threads have had problems loading when a lot of posts were deleted. The deleted posts cause phantom pages to appear because the server thinks the thread has more posts than it shows. If you ever see a thread that shows it has 300 pages but you can only get 299 of them to load, that's due to deleted posts.