I seem to be having an issue on my site whenever content is submitted that triggers an email to be sent. For example, comments on blogs (when the module is set up to email the owner every time a comment is posted); a moderated forum (everytime a reply is posted, the moderator gets an email); or just signing up as a new user (email to admin and to new user)... it's painfully slow and takes about 3-4 minutes before the page changes. The emails get sent out, and eventually the content goes through, but the slowness is unbearable.

I've checked the logviewer, but don't see anything that looks like an error around these times. Is there anything else that would cause this, or someplace else I can look to investigate?

I've seen the same issue. What happens if you test your SMTP settings under host? That's slow too?