I wanted to follow up with this post, as I feel it's a rather critical one. We actually ended up finding a solution to this issue months ago, however, I just haven't had the time to post until now.

Anyway, if anyone is having the slow posting issue like I mentioned in my opening post, this is most likely due to the fact that wpforo is telling your server to send a bunch of emails all at once, to all users who are subscribed to particular topic or category. This is why creating a new post can be PAINFULLY slow, and why editing a post is quick. Any new posts created will not be posted until all mail is sent out.

Solution: Install a mail queue plugin. This will queue your mail in smaller batches, rather than  having your server send out everything at once.

One thing I wanted to mention, is that this really isn't about whether you have a "heavy duty" server or not. Why? Because we're actually using a super fast WP dedicated server and the same issue persisted until we installed the mail queue plugin.

There are several free ones, and the one we use is this one: https://wordpress.org/plugins/smtp-mailing-queue/

I think this is a rather big issue, and one that could be probably be resolved with simply a mail queue feature built in. I also think that this should be mentioned by the admins here within the forums since many people seem to be experiencing the same issues (below are links to just three examples out of many). Instead of panicking about whether slow posting is being caused by a database issue as it's been often suggested, mail queue is actually the first thing that needs to be checked.

https://wpforo.com/community/how-to-and-troubleshooting-2/slow-when-posting-a-new-topic/paged/2/

https://wpforo.com/community/how-to-and-troubleshooting-2/posting-is-very-slow/

https://wpforo.com/community/how-to-and-troubleshooting-2/posting-is-painfully-slow

Anyone who has a decently sized forum with over a few hundreds users is going to run into this issue if the mail is not being queued or unless if you remove the ability to subscribe. I'm pretty surprised that this isn't even mentioned in the documentation.

The best and unique mail queue plugin for that reason is

https://wordpress.org/plugins/gd-mail-queue/

and not what you mention above. The plugin you mention will only create issues.

Bump!