how long does it take to synch across devices
I am seeing the iCloud Photo Library syncing getting faster in iOS 11, even with my backup phone iPhone 5s. 

Prior to iOS 11, it takes as soon as several hours, and up to days for the new photos to show up on all my devices. And at that time, it needs WiFi connection for the sync to happen. **The feature is designed the way that plugging into power and connected to WiFi triggers syncing. **

But in iOS 11, the photos even sync on cellular (but limited, eg. videos excluded). And the photos show up on all my other devices almost instantaneously - within minutes. (And my home and office WiFi is upgraded at all)

To conclude, to make the syncing faster:


Update to iOS 11 (it really helps with the photo syncing process)
Plug the device to power
Connect to good WiFi
Suspend other network heavy applications such as App Store Updates.