Mostly it does not come across as being that slow to me although I did see the odd and sudden slowdown where it looked like it was waiting for a server slot to become available. I don’t know what sort of contention for the server you experience of course nor how well your hosting copes with it.
I did see the same odd (and it really was occasional) sudden wait on all areas pf the website not just the forum page. Considering that the forum loads dynamic data that is also compiled specifically for the user loading the page and the rest of the site tends to be static data then it is clear that the forum page would take a little longer to display but I didn’t really notice much difference. Yours is also pretty light on the javascript files being loaded which came as a surprise after you saying you used lots of WP plugins.
I do not know how your mySQL is set up and I have no experience of optimising mySQL but if you have a high contention rate and lots of concurrent users on the site much of the time then you might want to consider switching to InnoDB tables if you are not using them now. Most hosts seem to always set up and default to the MyISAM engine which is fine most of the time, But when things get busy – especially if there is a lot of db ‘writing’ activity as one can experience with a forum, then InnoDB tables are better placed to take the strain. When myISAM needs to lock data for changes it locks the whole table whereas InnoDB only locks the table rows needed. But this would be a minor change on the whole.
The bottom line for me personally was that if this were my site I would be relatively happy with the performance bit would want to try and solve that occasional longish wait that seems to randomly happen.