I'm using a filter (https://stackexchange.com/filters/305027/sam#) to keep track of new questions within my area of interest. I have email updates enabled for every 15 minutes.
Since a couple of days, these email only come very sporadically. For example today I got an update that summarises questions posted several hours apart and sent about 12 hours after the oldest question was posted.
Update: After getting this email at 9:49 (local time), several more copies of the exact same mail arrived at 10:52 and 12:12 (both also local time)