Try using Chrome and see if it's better. I have been testing internet speed as well lately and the ISP service tech showed me that when using Chrome, I get the download speed I'm paying for, 100 Mbps. When using Firefox, the speed is half that usually. Both browser versions should be fairly up to date. I've also seen where some Windows update issues will cause a particular browser to slow down vs another and then the problem goes away with the next Windows update.
Seems this could be a never ending battle. Good luck.