I've been running DNN for many many moons.  It seems that only over the last year as our forums have grown (number of posts)  and we've upgraded DNN into version 5.x.x that posting has gotten so slow or hangs only when email is on.

If I turn emailing off, or if I remove the SMTP server from Host settings it speeds up.  So it obviously has something to do with the sending of mails when posting.

Has anyone else had these issues and is there a solution to this?  It really is causing huge issues with our site(s).

Anytime a site is active and has a lot of posts happening (especially if people subscribe to forums/threads very often), it is a good idea to use the Email Queue that is available in the module (that is what we do here). This is installed along w/ the module but must have a Scheduled task enabled by a Super User in order for it to be enabled in the module. When the queue is on, not only is the sending of the email moved to a scheduled task that runs in a separate thread but it also populates a send to list from this task as well. This offloading of the email population and send makes the posting process much faster. 

I did think this was the case and had turned on the scheduler for this a while back and what I found when this was turned on was the site would actually freeze (non responsive) or would kill the worker process at some point.  Are there specific scheduler settings that would work best?