ICloud – New Photo app – upload speed to iCloud photos really so slow

There are multiple absolute and rate limits on iCloud photos. See this article for details:

• https://support.apple.com/en-us/HT202299

Currently, if I had to guess, you hit one of the 24 hour limits and need do nothing but wait for the time to pass:

Maximum combined number of photos and videos from all contributors per shared album, per hour: 1000
Maximum combined number of photos and videos from all contributors per shared album, per day: 10,000

However, if you continue to have issues or are getting multiple notifications like your screen shot indicates, there could be a fault in the waiting loop code and you'll want to reach out to AppleCare to have engineering look at your account in particular. That isn't quick, but in the end usually works well if you are patient.

Solved

When browsing which picture to add to a mail (Gmail in browser, for example), there is a "photos" button down to the left

Clicking that, will show all the photos