Yep, same here. If anything, it's actually worse with iOS 15. I have an iPhone, iPad, and Mac and notifications are all over the place. Typically "Apple related" notifications (e.g., Messages) are delayed severely on the iPad and Mac after I see them on the iPhone. With respect to other Apps and their various notifications, the delay is measured in many seconds if not several minutes depending on the App and particular notification.
Like others, I'm unsure what Apple did along the way, but I began noticing these delays at some point in 2020. The fact they can't correct this widespread bug is absolutely pathetic.