Part of the problem has to do with email notifications. People can follow a thread or forum and receive an email whenever something new is posted. This makes it easy to keep on top of new content.
However, it's not quick to send dozens of emails. The site has to connect to the email provider, transmit the message, and then wait for the provider to send that message on to the recipient. This process can sometimes take a few seconds, and that time compounds with many followers.
To make this process quicker the site will now queue emails locally on the server. This removes the overhead of immediately connected, transmitting, and waiting. The server will immediately accept the emails and the post will be added.
This is only one part of the problem, unfortunately. With hundreds of thousands of posts, the site's database has become quite large. In the past, posts would be deleted to decrease the strain on the server. This is something I never want to do because old posts may still contain relevant content. Instead, optimizations and tweaks will be made to the database server to help speed posting and the entire site up. More information coming soon.