Welcome to the wonderfully unreliable world of iCloud. It is so glitchy that Howard Oakley wrote a utility called Cirrus, which you can download for free, and which is capable of “unjamming” iCloud. I use it regularly. For more information, see his website.
There is no “fix” for iCloud being slow that I know of. It is a well known problem. In my own experience I have had a file waiting to complete downloading for over 24 hours. The missing part of the file was 1Kb in size. iCloud will just jam completely at times. You can read this article with some information that might help.