I'm not totally sure that would have solved the problem anyway, but it was worth mentioning.
It's weird. I used to get notifications immediately, every time. Starting around a week ago, I noticed long delays. Sometimes several hours would elapse before I would receive thread updates. But almost always now they arrive at least 15 minutes late. It's Gmail, so I don't believe it's a provider issue, especially since it's been going on so long. Plus, I haven't noticed it with other incoming emails. I just sent a test email from another email provider and received it right away. So....