The forum speed is returning to normal for me. I still think that downstream caches got junk in them after the problems of a day ago. This caused the requests that did get back to the forum to confuse the forum software totally, particularly with "reply" type pages.
As the junk in the caches is clearing, everything starts to work again. No real evidence, but that is my guess.
Whatever that plugin did that caused this problem, it was pretty bad. If it was my site, I would even change the admin forum and WordPress passwords in case the plugin included some malicious software.
Richard.