I have the same problem, 2 days ago I started my PC and the internet was just unbearably slow compared to what I'm used to. Speed tests suggest that my internet is fine, and on other devices, the internet works just fine too. But I'm unable to play games because my ping is too high and even youtube/google struggles to load fast.
And the thing that's weird is that I'm directly linked to the router via an ethernet cable. I tried both, wifi or ethernet, its the same. Also restarted router and PC multiple times. (Also tried my 4G hotspot which usually works perfectly fine but it was even worse)
Another thing I noticed is that my ping goes up and down randomly, it'll be 30 for 10 seconds, then jump back to 400 for a few seconds, then to 3000 for a few seconds and repeat randomly.
What's even weirder is that when I try to download a game on steam for example, it downloads really fast, but when I hop in a game, the ping fluctuates and is almost unplayable.
So I think something is really wrong with my PC itself, something changed and I have no clue what changed.
(Now that I think about it, it might have started after the day after I installed Valorant. Might have nothing to do with it tho.)