iCloud photo sync is ridiculously slow

Keep in mind that iCloud is designed primarily for mobile devices, and since syncing uses the battery, and since lots of people are very anxious about battery drain, the iPhone will avoid syncing when not plugged in. And since syncing uses processing power, and people hate slow-downs, the iPhone will avoid syncing while you use it. And since syncing requires bandwidth, the iPhone will avoid syncing when the internet connection is weak.

So, if you want faster syncing, then find good wifi, plug the phone in, and leave it alone. All this applies to a lesser extent for the Mac.

When I'm in a hurry, I use AirDrop. AirDrop does not use iCloud. So if fast transfer is your reason for using iCloud, you may need to rethink that. It's not the primary intent.

Nevertheless, I'm sitting at a cafe with OK wifi, and I deleted a picture on my Mac, then opened my phone, and I saw that picture disappear from the iPhone about 5 seconds later. There was no outside power.

So first, how is your storage on your iPhone and on your Mac? Apple recommends keeping at least 10% of your storage free, otherwise some things will take a long time, some app will do crazy things, and some processes won't work at all.

Have you tried a Forced Restart?

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after the Apple Logo appeared indicating a re-start. 

Have you tried operating the Mac in Safe Mode? This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

 Start up your Mac in safe mode - Apple Support 

When you add a picture to your Mac, what do you see in Photos at iCloud.com? Does it get added there immediately? How about the other way?

Let us know about those tests…