The Reply function is VERY slow and acting strange
 

Lately, my forum is VERY SLOW to receive a reply post to an existing thread, but navigating from thread to thread, forum to forum isn't slow at all. The site is on a dedicated server and its the only site operating on that server. There are 150k+ threads, 2.2+ million posts, and 46k members.

Today, I timed how long it took to submit a reply. It took longer than 2 minutes at a time when the forum wasn't busy. While I waited that 2 minute period, I opened another tab and navigated to the thread I was posting to. I discovered that my post had been successfully submitted yet the original browser window didn't reflect this. It actually was still showing that the site was still in the process of accepting the the submission of my reply.

Any ideas what could cause such a slow response?

Edit: I just attempted this again with the same results. Server load is 0.90-1.39 right now.