Web Application Firewall is a complex piece. Complex pieces are complex because they have to deal with complex set of situations. So while it's possible that the firewall has changed, it's more likely that the firewall's environment has changed. That could be higher normal traffic volume or malicious attacks. Or the firewall appliance at Sucuri could be shared by multiple tenants and others on the same firewall are being loaded or attacked, etc.
And there is something else by the way: There are shortcut links sprinkled around the forum. Those links seem to be unexpectedly slow. For example:
The shortcut link to Steve's latest post in this thread pointed to
https://www.whatsbestforum.com/posts/637159/. Clicking on it takes you to Steve's post here, but the thread page itself usually takes 10+sec to start appearing. Interestingly once the dust have settled the final URL in the browser is
https://www.whatsbestforum.com/threads/forum-is-really-really-s-l-o-w-maintenance-going-on.30297/page-2#post-637159. So the generic post number got turned into a specific thread and a specific page in that thread before scrolling to the post in question. Looking at the traffic flow in Firefox debug mode shows the browser has been told to go directly to the exact latter URL via an HTTP 301 redirect. So all the work was done by the forum software on the server side, which actually takes 10+sec to spit out the HTTP redirect. Once the browser sees the redirect, things start happening quickly. Click on the two links above and see the difference. Now if you go through the Forums ? Forum ? Thread ? Page navigation one at a time, the individual step appears very fast every time. Other shortcut links elsewhere seem to be done with HTTP 30x as well, with similar delay.