I'm experiencing a problem with the site for several months now. I used to be able to download with speeds ranging from 600 to 700 kb/s and for a long time now it's gone down to 25, max 35 kb/s making it impossible for me to download larger titles and a pain to download even smaller ones. This occurs no matter what time of day or which day I try to download, so I'm guessing it's not a server load problem, unless the servers are overloaded 24/7! It's also not a problem on my side, no other site or any other kind of download has slowed down for me, only GOG. Anyone having the same problem or knows what's going on? Thanks!

A bit more info is required. Are you using the downloader or downloading through your browser? Try using the other method (browser if currently using downloader, downloader if currently using browser). There were a few problems with some antivirus blocking the downloader and/or the CDN gog uses, but can't recall which ones atm.
Downloading through the browser should give you ~1MB/s, or line cap, whatever is lower. Downloader should be faster than that.

Downloading through Mozilla Firefox on Linux Mint, so I can't use the downloader (only works on Mac and Windows). I download from here because my Windows PC never goes online, and just move the stuff there once done. The browser always used to give me line cap (600-700) until one day it just didn't anymore, and only on GOG.com not on other sites. I am not running any active antivirus. Meant to reply but instead added a new post, sorry... see above. :)

Any addons on mozilla? NoScript, AdBlock+ or anything similar?

Point is that the files are not stored on www.gog.com or secure.gog.com
They use a Content Delivery Network (CDN in short), I think Edgecast, but the free games may be on amazonaws (or however the server is called). If running without addons solves the speed issues, then you need to whitelist the domain(s) and you are set.

Just tried downloading will all addons disabled, the download still caps at 25 kb/sec, same as before.

Trying to download 99 levels from hell gives me a link in wpc.11df.edgecastcdn.net
The best I can offer now is the usual network check to see if edgecast is reachable and unthrottled, though it's been too long since I had to do those checks in linux. Alternatively, send the support a message, they should be able to look into it and hopefully reply to you soon(ish).

P.S. Can you also try it with another browser, just to make sure that it's not mozilla that's acting up?

--- edgecastcdn.net ping statistics ---
11 packets transmitted, 11 received, 0% packet loss, time 53493ms
rtt min/avg/max/mdev = 267.685/270.155/277.230/2.963 ms

--- gs1.wpc.v1cdn.net ping statistics ---
12 packets transmitted, 11 received, 8% packet loss, time 54630ms
rtt min/avg/max/mdev = 272.022/277.725/282.443/2.815 ms

Both are terrible results :) so yes, the connectivity between me and edgecast is quite bad.

What about traceroute? That should help identify the server that hiccups.

traceroute readout tells me that all of the hops are slow, starting from ~50 ms and gradually going up to ~300ms

Some ISPs limit the download speed when they "think" the user/client is downloading illegal stuff, like for example, via Torrent or P2P. And, in many cases, they are wrong... This is a borderline practice done by them, very unknown by many people, but many ISPs do it. This could be the case for you.

In my case, I have no problems to download from GOG (using browser:Firefox, and the GOG Downloader). I can download at the maximum speed for my bandwidth, even from Chile.

In that case, I'd suggest calling your ISP and complaining :/ Should be ~9 hops with a top latency of ~100ms. It is possible that one of their systems is down, thus they are routing you all around the globe. And as I recall, edgecast is used quite a lot, thus you may encounter slow speeds with other sites as well.

I'm aware of the throttling some ISPs do, I think mine doesn't (I don't think any ISP in Greece does), besides I am always downloading things that could be considered "suspicious" (ex. torrents) and those go up to full speed. And no, not downloading anything else when I'm trying to download from GOG. :) I have also tried to download with forced HTTPS, and in that case they wouldn't even be able to tell what I was downloading.
Perhaps that is the case. It's weird though because I'm kind of download-crazy and I have only encountered speed problems here, nowhere else. And only for the past three months or so, before that I was downloading at max speed from GOG as well.

OK just tried your second suggestion and begun a download from a freshly installed Google Chrome browser, but it still gives me the same slow speed, so it's not browser related either. By the way, thanks for all the suggestions, I don't want to sound ungrateful. :)