You may be right but have you tried the downloader? The default setting is for four download connections but you can have up to ten, which is bound to be faster than the single connection you have when you download manually.
The number of download connections I'd have using the GOG downloader wouldn't matter because I only have a certain amount of bandwidth available to be filled. I get 1.4 MB/sec download speed from GOG, which is as much as my connection will support. To get more, I'd have to have a second network cable installed and run from my house, then get a second Internet account from my ISP.
If I used the GOG downloader and had 4 connections going at once, each would be 0.35 MB/sec because I only have 1.4 MB/sec total downstream bandwidth for all connections.
Don't confuse reporting methods.
1.4 MB/sec = 1.4 Megabytes
Since there are 8 bits in a byte,
1.4 MB/sec = 1.4 x 8 Megabits
per second (Mbps) = 11.2 Mbps
When you download from a website, your speed is usually reported in bytes/sec (MB/sec or KB/sec).
When you subscribe to an account at an ISP, or use a bandwidth-measuring site, they report the download speeds in bits, or megabits.
So a 5 Mbps connection from your ISP would actually give you download speeds of 5/8 MB/sec, which is 625 KB/sec.