How many is a "large number"? If there are 50 or less (remembering off the top of my head here) we send the notifications immediately, but if there are more we queue the notifications to be sent by a background task. If we are sending notifications immediately, the speed of your email server will make a difference, but if we queue them then emailing is not likely going to be the problem.
Do you use Elasticsearch? It's possible this is slowing down posting if the newly posted content has to be pushed across the internet to a slow ES instance.
Any custom applications or plugins? Many are designed to do X when Y happens, which often includes submitting a new post or topic.