The answer to your question, "Is Adobe Creative Cloud notoriously slow?" is no, it is not. This is true of most commercial grade cloud applications.
It's important to note that upload speeds to a cloud server will be substantially slower than downloads from that same server. This is just the way that ISPs provision their network connections and there is nothing you as a user can do about this other than to pay more for a faster network connection.
You can check your upload and download speeds for free by going to
https://www.speedtest.net/
There are, of course, other factors that can effect how well a cloud application will work for you such as the CPU performance on your computer, amount of RAM, whether you are running solid state drives or spinning hard drives and so on. But since your question focuses on very slow uploads, the first place to check is your upload speed and performance.
It is always possible that Adobe may be dealing with a DDoS (distributed denial of service) attack at the very time you are doing these uploads. Not terribly likely but if speedtest shows that you on a very fast line (e.g. fiber), checking with Adobe support to see if they are having outages would be the second step.
I hope this helps.