Lots of helpful people here @pluhmen @ross @otto42! Thanks all!!

My use case is non-standard :-) For FediTest.org, I'd like to deliver outgoing ActivityPub messages asap, so my tests don't take longer than necessary. I've attempted to invoke wp-cron.php manually but it's not sending immediately ... 15 times with 1 sec delay often works, but not always. As soon as I open any page in the browser, it goes through, however. It's the unpredictability that I'm trying to understand ...