Internet slow during web-browsing

[JohnnyB]

hello everyone!

I'm using Linux Mint for years now and I also installed it on my mom's desktop and it connects directly to the main router with Ethernet cable. She is using it mainly for browsing, youtube, facebook etc.
Everything was fine until a week ago when after some OpenSSL updates her web browsing became extremely slow. By slow I mean that for example the youtube mainpage and facebook also needs a minute or so to fully load and it can't play youtube videos not even on 144p, before that it could easily play videos on 1080p and the websites loaded in a second. The problem remains both with Firefox and Chromium. But here's the catch, when i try to download something from a website, let's say from www.download.com if I'm downloading some app with a bigger filesize, first the download speed is very slow, around 3-5kbytes/s but in about 30-40 seconds it climbs to 15MBytes/s and it stays on that maximum speed till the end. So I don't understand what's the cause of the problem. If the router or the cable would be the problem, then it would not download files with full-speed of my net-provider, eventually it climbs to the maximum speed but it takes time and it works only when downloading bigger files, for browsing it is horribly slow, basically unusable. When in facebook i try to send a message, i write something and then hit enter, it says that the message is sending but it does not refresh the status to "message sent". It only says "message sent" if I refresh the whole webpage. Facebook messenger also says every 5 or so minutes that "there is no internet connection" but only for a moment, after that the connection comes back.
I tried to connect my windows laptop on this same cable and the speed is lighting fast, every site loads in a second and there are no speed problems at all.
I'm not sure that the OpenSSL updates caused the problem but I don't know what else could because other than some updates, I didn't change anything.
Also it's worth noting that this problem started when the Mint version was 20.3, so I thought maybe if I update it to the latest version, maybe then it will solve this problem. So i updated it to 21 (Vanessa) but the problem remained, lastly I updated it to 21.1 but the problem still remains.
I really don't want to switch from Mint to Windows because Mint served me well for years. I hope that there is a solution to this.