windows 10 – Some process is making my download speed 95% slower

I have fibre direct to my router, i get generally 850-950Mb/s download and average 120Mb/s upload.

Running Windows 10, 6700k, 1060 6GB

My speed often goes down to 2-50Mb/s sometimes up to 120Mb/s and to fix it i would need to either restart my PC or use JRT.exe which auto closes loads of processes that are not required. Using JRT bring my speed back up to normal instantly. I found that when running rocket league (which starts steam also) my speeds drop, but force closing steam, and rocket league does not help. Only what’s mentioned above.

I do not think it’s the network card, i have troubleshooted that a lot.

I cannot find which process is making my speed so slow, what is the best way to diagnose that?

Thanks