This happened to me 2 times now and GOG.com hasn't responded to my problems about the downloader causing my computer to slowdown really bad.

yeah can happen if you download at 6mb/s , people need to be able to throttle down the speed.

If that is the extent of the complaint you filed, then no wonder they haven't responded. Provide details: what exactly do you mean by "slowdown", is it just slowing your internet speed (not really surprising) or the function of other applications? If it is slowing down other applications, have you checked the RAM and processor usage? What other apps do you have running at the same time (one or more could be conflicting with the downloader)? How do you have the downloader configured? Have you tried limiting the number of concurrent connections? Most importantly, what are your system specs? If you are running on an old, weak machine, then there is really no complaint to be made, but if you have a modern, competent machine, then this might something worth researching.

No what you are not understanding is that when my download was finished my computer suddenly became really slow when it started verifying all the files that I downloaded.

So it's not the downloader, but the hashing function, which especially for games between 1.5GB to 2GB will also have your antivirus scanning the huge file.
Normal operation, hashing is resource intensive.

It's not that I or anyone else is not understanding, it's that you didn't tell us that. All you said was the downloader slowed your computer down, with no specifics at all. If it is slowing down while verifying, that adds another question to the list of questions I already posted: How big a file were you downloading?

Also, how fast is the hard drive you are downloading to? Has it been defragmented lately? Etc.

Do you use Avast by any chance?