Thanks for both your responses. I checked speedtest.net, and while my download speed is fine (90 Mbps), my upload speed is only 0.54 Mbps [Edit: I tested again and got 1 Mbps], which is rather slow, although it should still be possible to upload almost 2 GB per hour.
I read elsewhere that one aspect to consider is throttling on Apple's side:
Extreme slow upload on iCloud - Apple Community
So, perhaps throttling and slow upload speed combine to create this issue. Although I don't fully understand.
Someone else mentioned things to try like resetting PRAM:
iCloud extremely slow on superfast network - Apple Community
Anyway, things have gotten slightly better. It looks like about 2 GB uploaded over the last 12 hours. My iCloud Drive upload is now at about 4.5 GB out of 30 GB. So, perhaps I will only have to wait about a week, although it seems like an unfortunate use of electricity to keep my computer on 24/7 for a week for something that should be faster.