Networking – Internet slows slows down when downloading large files for no apparent reason

In the past I have had the same effect with my ISP (in a rural area too, though this is but a coincidence).

The root of the matter was that he implemented a bandwidth limitation on port 80 only. This was especially obvious with speedtest.net, where the initial speed would peak, then decrease to less than half of its peak value.

I discovered by pure chance that this did not occur on OpenVPN, where I managed to obtain the peak speed value during the whole speedtest.net test. This was made possible by the fact that the site I connected to (my work site) has a very nice, very fast, large-bandwidth connection.

Alerted by this, I tried a large file transfer via scp, and, lo and behold, I achieved the same large speeds as with the OpenVPN, rather than the lower http speeds.

You may try the same, and see whether a bandwidth limit is applied on ports 22 (scp) and 21 (ftp). It is most illuminating to use files already significantly compressed like pdfs, since this will rule out the incidence of compression per se.

Though admittedly sloppy, these bandwidth limitations are still effective, since most people use the Internet only for downloading Web pages.

EDIT:

Strictly speaking, there is a way to test this: if you control the VPN server, you may stop any other activity on the server on port 80, and start listening for VPN connections on port 80; you will also have to modify the port you connect to on the client's program. Now, if your ISP is limiting bandwidth use on port 80, the VPN should clock at exactly the same speed as the non-VPN connection.

I found the solution: change the Energy Mode of Windows 10 from "high performance" to "Balanced". Somehow the "high performance" mode was only using one core of the CPU.