I mentioned the difference in the results between single and multithread, but they say that i shouldn't "bother with it", as if to say that these value mean nothing. Obviously I tried contacting my ISP about it, and while this issue has been resolved a couple of times already, it's always coming back after a few weeks or so.
It was here at first that I learned about the difference about Singlethread and Multithread speedtests, and after going back to other sites which also showed the same options, I realized that while my multithread tests average about the expected 90mbps, singlethread gave me way lower results, with huge variation even within a few minutes difference, but still way lower than the 90 (usually it's been about 5 to 20 mbps, but this number is getting always lower).
I started having doubts when on (i'm just using Twitch as example as it's a renowned streaming website) I would end up seeing streams buffer, way more than usual, and definitely not something you'd expect of a 100mbps connection, but my Ookla speedtests always gave results of up to 90mbps, which is what effectively arrives to my router and what it appears on the router page itself, so at first I thought it was some issue with Twitch itself however, after finding and similar sites, my speedtests started getting more and more like this one: However, for the last 18 months or so, i've been having recurrent issues about the effective speed of my connection. I live in a rural area in Italy and haven't had access to even the most basic ADSL up to 2016 or so, but then things changed and I had the luck to get a VDSL2 connection, a 30 mbps line which got upgraded to 100 a few months later.
For the last 2 years i've been having a difficult love/hate relationship with my ISP.