I have a topic that has grown in size...
The issue is related to email sending service in your hosting. If a topic is large, there are a large number of subscriber. So when you're posting a new reply, it should send emails to dozens of users. This step takes the 90% of the time. Some hosting emails sending services are not fast, and they make such delay.
@robert Thank you Robert. Any recommendations to speed things up? One thought would be to unsubscribe everyone from the topic. Is that even possible? If so, how could I unsubscribe everyone from a specific topic?
All emails are being sent through WordPress wp_mail() function, which is based on the hosting server email sending functions. So you should use a WordPress plugin which supports email queue feature: https://www.google.com/search?q=wp_mail+email+queue