I installed OS10.10.3 on my iMac and started moving my 27,000 photos from Photo to iCloud. I let the program run all might and by the next day I have moved 10,007 photos to iCloud. It appeared to me, however, that nothing was working and no additional photos were being added. I restarted the computer and the Photos program and attempted to download 88 pictures from my camera. Photos started up okay, I saw the 88 thumbnails, but the program would not download any of the pictures. I then attempted to quite Photos, but received a small box on my desktop that said something to the effect, "Closing your library ..." I waited several hours, but the notice did not disappear. When I attempted to force quit Photos I was warned that to force quit at that time might lose any changes. I waited several more hours, nothing changed. I force quite Photos and restarted the computer.
I then restarted Photos and it was exactly as I had it before (by that I mean that nothing was lost). I did find that I had an additional 50 of so additional photos uploaded to iCloud. I then started Safari and logged into iCloud. With both Photos and iCloud open, (and iCloud open to photos) I then scrolled down to the bottom of the Pictures section on iCloud. At the bottom of that screen is a counter of the number of photos you have uploaded to iCloud. Every 10-20 seconds the spinning timer icon appeared and my iCloud photo count would be updated! I was adding approximately 12 photos per minute (or around 700 per hour). You only get the spinning icon for about 5 minutes, after that no more updates. If you mouse click the refresh button on Safari it reloads the page and starts the counter 5 minute cycle again. I have let both programs run for about 6 hours now and it has uploaded an additional 4,200 pictures. Only 18 hours to go to upload the other 13,000 photos :-( (I'm not complaining - at least I'm making progress).
I did notice the following while Photos and iCloud are running. (1) Some other programs would load very, very slowly. It took Messages around 2 minutes to show up on the screen and another minute to show me one new message. (2) The mail program will load, start to download new mail, then stop downloading mail (it appears to stop doing everything). You have to quit Mail, and then Force Quit mail, to totally quit the Mail program.
I also noticed that the newly transferred photos to iCloud show up both on my MacBook Pro laptop and my iPhone. I also found that both my laptop and iPhone work fine, both loading programs and the ability to receive email!
I am not a very learned hardware person. My totally uneducated guess is that my iMac is using up almost all of it's computational power getting the photos ready to upload -- and then actually sending them to iCloud. I find it strange that that the upload is progressing so slowly. I have FIOS with 75 Mbps of both upload and download speed, yet as best as I can figure I'm getting about 1 Mbps upload speed. (Here's my admittedly shaky math: 700 photos / hour TIMES around 5 Mb/photo = 3,500 Mb per hour of uploaded photos. Divide that by 3600 seconds in an hour = 0.97 mbps which is the average upload speed for the hour)
My point is that to try opening ONLY Photos and Safari, and in Safari only log into iCloud so you can see if you are actually uploading pictures. As far as I could tell, that was the only way that I could actually see that I was making progress.