iCloud sync is painfully slow and unreliable

Hello,

I pay for iCloud 2TB with the intention to be able to sync all my files across my units; iMac, MacBook Air, iPad and iPhone. However, the sync isn't smooth and seamless at all.

It takes days and more to sync files added or downloaded on my MBA to iMac and vice versa. I expect them to sync immediately. Why don't they?