It looks like the app is creating a large number of subscriptions to the bookings.byDateTimeRange publication. By default, each subscription is set up one at a time, so it has to wait for all of the previous subscriptions. The best solution would probably be to reduce how many subscriptions are created. Another option is to call this.unblock() so they don’t force other subscriptions to wait on them, though there are some bugs with unblocking publications.
In development, you can use montiapm:debug to record and view traces of publications.