You mean slow over Downloader right? Can you tell me what's your average download speed when downloading game from GOG via browser VS downloader? (please check for game, not for addons). Downloader shouldn't be any slower than browser as it's using exactly the same file request, and because files are downloaded in concurrent sessions too, as long as your connections is fast enough, you should be actually getting noticeably faster downloads.

PS: it would be helpful if you could post the Downloader version you're using + number of concurrent connections selected (see Settings menu).

Ok, but without the other information I've mentioned in my previous post, it's really difficult to nail the cause of your problem :). Could you please post
1. average download speed when downloading any GOG.com game via web browser (it must be a game installer, not a bonus goodie)
2. average download speed when downloading the same game as in #1 but using GOG.com Downloader (you might want to restart it or do pause/resume first... also make sure there are no other downloads running on your computer / network)
3. number of concurrent connections selected in Downloader Settings

Also, it would help if you could also post your connection speed test results (from speedtest.net for example) so we would have a general idea what your connection should be capable of.

Stupid question but I assume that your network is much faster than 6mbps, right? :) . It might be the case that the reason for Downloader speed not exceeding 650 KB/s (even though you have 6 concurrent connections vs browser that uses just 1 connection) is the distance to our nearest CDN server.

What I'd recommend is to try changing the number of concurrent connections to 4 and then 10 and see if you can achieve any better results with any of these (just remember: pause and resume your download after for the change to take an effect).

(the idea behind a increasing number of connections if kind of obvious. As for decreasing the number of connections, it actually does help sometimes as on some network or hardware configurations it does happen that too many connections throttle the entire download - too much I/O I guess)
Try restarting the Downloader - we do have a known bug that the Downloader sometimes stops at the very end. It should help. It will be fixed in the new update.

If it doesn't help, please do check the The Witcher download folder in your GOG.com Downloads folder and see if there isn't one or few files exceeding size of all the other chunks (the chunk size for this game is I believe 10240 KB or less, never above that).