If you search the forum, you will find there are many many similar threads as yours.
Unfortunately GOG treats many of us as second or third class citizens and gives us a lousy download speed.
I don't recall whether they were using the Fastly CDN before the middle of last year, when we had a kind of crash here, where you weren't able to download games for a while. But when downloading became available again, many of us have had an extremely slow download speed ever since.
At least, the slow speed is limited to a single thread. So you can get around the issue somewhat, if you download with a program that uses multiple threads, such as Free Download Manager 5. Alas GOG also seem to impose a thread limit, so you aren't likely to get a really great speed, but perhaps good enough.
Some have reported getting around slow downloading by using a VPN.
It is also possible you might fare better with Galaxy.
Another downloader you can try, is Heroic, who have reportedly been in talks with GOG.
NOTE - Many folk around the world have this issue, even in similar locations to folks who don't have the issue. Most folk don't seem to have the slow speed issue, though they have noted a significant drop in speed.
If I was to speculate, I would say it is all due to GOG cutting costs with servers etc.