Hey, I'll try to provide as much information as I can, and everything censored can be uncensored on request.
Since the past 4+ weeks, I get daily issues with many websites becoming unusably slow. What I experience is that the website either loads forever, or it loads within a few MINUTES. For example, a website with a few images on it, takes 2 minutes to load a 20 kb thumbnail! Or I once tried to watch a 3MB video on Discord and it took 5 minutes to buffer! (See image 1 and 2). It feels like using dial up internet on the modern web.
The strange thing is that all speedtests I try return that I get my full 250 Mbit/s connection without any issues while this happens. Eventually I realised that the one thing the slow websites all have in common is that they all use Cloudflare.
So I went to https://speed.cloudflare.com/ and I noticed that whenever the websites become unusable, speed.cloudflare.com also gives me abyssmal results. Single-digit download speed instead of the 250 Mbit that I get on every other speedtest. And I get up to 50% packet loss, if the packet loss test even works at all (most of the time it just failed without a result). Ping and jitter are also high, I once got a jitter result of 400ms. I repeated this test at various times, and it's only the cloudflare speedtest that is having terrible results, all other speedtests achieve the max speed 24/7.
I tried an MTR to various websites that were having issues, and I noticed that they all routed me through
CLOUDFLARE.edge6.Dusseldorf1.Level3.net
CLOUDFLARE.edge6.Dusseldorf1.Level3.net
which is giving me a 50% packet loss in the MTR.
The slow websites usually don't occur in the morning, they start in the afternoon and evening, so it's probably related to load.
My neighbor (in the same building) is having the same problem, using the same ISP.
I contacted my ISP and they said my connections is great and there's nothing they can do about it (even though they're our Tier 1 network).