Could it be you're on a slow server or your access is with a slow provider?
I'm setup for 25mbps but the most I can download at is around 1mbps.
Some sites are stingy on the amount of bandwidth they allow you.

Are you using the GOG Galaxy client to download and install the game?

If so, you can do these things (both, if necessary):

1. Exit GOG Galaxy, and log into your GOG account with your web browser (Edge, Firefox, Chrome...) instead. Go to
https://www.gog.com with your web browser and log in with your email address and GOG password. It may trigger two-factor authentication in which case you need to get the code from your email box.

Then in the upper banner of the web page, hover the mouse (don't click!!!) on your name "edfightermage" => click Games => write Witcher in the search box => click on the "The Witcher 3" => click on "Download offline backup installers" and download all the 10 game installer files and the 1.32 patch to your PC with your web browser.

By default the files go to your Download folder, so after you have downloaded all the 11 files completely, in that folder click on the executable setup file to install the game, and after the game is installed, run the patch file to update the game.

Then you can play the game already (without Galaxy). If you want to import the game installation into Galaxy, you can do that too, from Galaxy. No need to redownload the game with Galaxy, you already have the game installed on your PC.

2. Especially if the web browser download is slow too, contact GOG support and tell them that you get poor download speeds from GOG servers. You can test what you should be getting at best by running the internet speed test at e.g. https://www.speedtest.net (click "I Consent" and "Go").