7 was a good OS and I liked it. 8 was different, unstable and messy. I never liked it, and only ran it for a very little while. 10 is different, but it seems to be a good solid OS. Once you get used to it, it's not messy like 8 was.
When machines are running slower - and all other things are equal (hardware, OS, anti-virus, adware, etc.) - I like to reinstall the OS. Basically reset the machine and get everything back to it's baseline. I go as far as to reformat the drive and start from the bottom up, and that usually makes things run faster.
