troymccann the reason I mention this is that by default Flarum sends emails and notifications in the same request that stores the post to the database. So the more people you mention, the more people are subscribed to the discussion or the tag, the longer it takes to dispatch those emails.
To resolve this, every community that reaches a certain level of activity, will need to consider enabling a queue. When Flarum has a queue enabled, email sending (for instance) is handled outside of the request that stores information to the database. The queue runs on the server and continuously looks for new tasks to process. Bringing these tasks outside of the user request speeds that request up tremendously, but usually only noticeable when the community is reaching a certain level of activity.
Enabling a queue (like glowingblue/redis-setup allows) is one solution. A lot has been said in this discussion about queues too: https://discuss.flarum.org/d/21873-redis-sessions-cache-queues
Another cause might be that your email service provider throttles you rather quickly and thus email sending is delayed. As this runs in the request of the user (without a queue) this can affect the loading time of that request.