WHY IS ICLOUD PHOTOS UPLOAD SO SLOW?

It is not just an upload, it is a syncing service. When you start syncing a Photos Library with iCloud Photos the library needs to be merged into the existing library in iCloud. This requires a check for duplicates and solving conflicts between duplicates (albums and folders, metadata, adjustments). And it will be much slower, if a library is large, the computational load is not just proportional to the number of items to merged into the existing library. And it ill be very slow, if you are having items in your library, that cannot be uploaded, because they are incompatible with iCloud, for example referenced files or items in a legacy format. be sure to remove all PDFs, or audio files, and videos with a codec that is no longer supported from your Photos Library and consolidate referenced files into the library, before you start syncing it with iCloud. This will stop Photos from wasting time on items that will never succeed uploading to iCloud. How to Weed out Legacy Media in Photos for Mac - Apple Community

Use a Wi-Fi connection for uploading, not a cellular Hotspot, and disable all antivirus filters or firewalls, that could be blocking the access to Apple's cloud servers.

No one here can answer why. Only apple can, and they are not here.

Leave your mac on 24/7 connected to power, sleep disabled (I use the caffeine app) and photos open.

For reference my small 50GB 5000 item library took 48 hours, but that was to an empty iCloud, so no comparison between cloud and library needed.

I feel your pain. iCloud has always been really slow for me too, with the upload speeds being unusable at times. I have 100 Mbps download and upload speed connected via ethernet cable, and on iCloud I get upload speeds around 3 Mbps, which is unacceptable.

It's so slow that it nullifies the point of the service, which according to Apple is – "to keep your most important information—like your photos, files, backups, and more—secure, up to date, and available across all your devices."

With such slow upload speeds, it's impossible keep larger files up to date and available across multiple devices. People have been flagging issues with iCloud speeds for a while now, Apple needs to address the issue (I'm aware this is the community board, but Apple does keep track of the issues people discuss here, as indicated by the "Apple recommended" tag on some posts).