I had the same thing with my previous internet provider, I even set up iperf3 test to confirm that and it was clear when watching anything from youtube. It was so bad that it could barely keep up with 144p on YouTube and when speed test was running i could easily watch anything in 4k, but imidietely when test was finished everything was back to snail speed. I ended up with doing download only speed test in loop from cli version, and even with this extra traffic everything was much faster and with better latencies...