This sounds like the old Netflix issue where an ISP gets hammered during certain hours. I will not duplicate the web on this or how the ISP's pipe to the Internet gets maxed out.
And no, an ISP should never overprovision. This can royally upset the entitled among us.
Nothing seems wrong here. Same old story.
I recently moved to my girlfriend house and she has her own internet service (60 mbits/s) so I dropped mine. When I was going to her house before, we were getting some buffering when streaming stuff that worked fine at my place but didn't really make a big deal out of it since I wasn't there often. I just thought it was the wifi.
Now that I moved there, I realized that at night between 8 and 10 all downloads become reallllly slow suddenly (100-300 kB/s). Its the same from my phone over wifi, its the same on my computer on wifi and its the same from my computer with a wired connection. At first I thought it was the site I stream from even if its usually extremely fast but it isn't because I also tested a bunch of test files to download on different sites that are super fast outside of that time period (20-50 mbits/s) and I'm getting the same results for all (100-300 kB/s) and on top of that I also tried the same links that are super slowed down on my phone using my data plan and I'm getting even more than 50 mbits/s on all tests. So the sources are not the problem.
At that point I'm thinking that this is just a crap ISP and they are throttling my connection because all of those links are working fine on my data plan but what I don’t understand is that during that time period when all downloads become super slow, if I do a test speed on speedtest.net or on fast.com for instance I'm getting very good results (between 30 and 60 mbits/s). I also tried a very popular torrent and it was downloading fast. How can all my direct downloads be super slow but my speedtest be totally fine? is there something that I’m not getting? Can my ISP restrain my connection selectively like that?