Saving a comment takes extremely long when a lot of users watch the space

We have a space that is being watched by 527 users. When you comment on a page it takes Confluence about one to 5 minutes until the user get's confirmation that the comment has been submitted. Until then he sees a spinner.

I've looked into the code and what apparently takes so long is creating the PreRenderedMailNotificationQueueItem Objects for the 527 users. This should be done asynchronously as it is not acceptable for the user to wait minutes until all the notification emails are build and rendered.