Why is iCloud so slow to sync Microsoft Word documents between my iMac and Macbook Air, sometimes 30 mins

Why is iCloud Drive so slow to sync Microsoft Word documents between my iMac and Macbook Air (and vice versa)? My Internet connection is fine. iCloud Drive used to sync a lot faster, than this. Both devices are running Yosemite 10.10.3.

I always save and close the Word document in iCloud drive, on whichever device I am working (say the Macbook Air). I even quit out of Word just to be sure. Then I switch to iMac and open up iCloud drive and the document is not there. It also happens the other way around. Usually takes about 30 minutes to sync. Anybody know how I might fix this? Or what is the cause?