Slow and unreliable Photos sync to macOS

Apple Photos seems to sync reliably and quickly on iOS devices (iOS 11, iPhone and iPad) and seems very hit and miss and quite slow on Mac devices (older iMac/Sierra and Macbook/High Sierra). Is there a way to force the macOS client to download photos from iCloud?

Currently get statistics from my Photos library and "downloading" message. Even after 24 hours, no downloads show up.

Tried re-starting the Photos app and the entire system. Tried unchecking iCloud sync in the Systems Preferences app. Tried changing Wi-Fi connections. Tried connecting with and without VPN.