I am not aware of any settings to control the speed of iCloud uploads. Other activity on the computer might cause the upload process to pause or slow down so perhaps letting it sit without using it might allow it to run faster.
Also, you may already know this, but many consumer internet connections have a much slower upload than download speed. You could have a 500 Mbps download speed but only a 20 Mbps upload speed. Are you saying that you're uploading about 1,000 to 1,200 photos per hour with an average file size of 10 to 12 MB? That would be about 22 Mbps which is pretty normal for a mid- to high-speed cable internet connection but would be slow for a fiber connection. If you're saying that you're uploading about 10-12 MB total per hour, that's about 23 Kbps which is very slow.
Out of curiosity, I'd perform a speed test on your Mac to confirm how fast it can go.
Also, be mindful in case you have a data cap. Some ISPs limit your data transfer and can get pretty expensive with overages. And you might also reach out to them in case they're throttling/traffic shaping. Sometimes you can ask for more upload speed and they'll give it to you.