Excellent summary by WeeJ. Should find its way into the FAQ file at the top of this section.
Answering the one bit he didn't address: I have two pretty-much identical Lenovo laptops, one with W7, t'other with W10. The W10 one runs slightly faster than the W7 one; both had a clean software install within a couple of weeks of each other. Only other difference (which I would guess makes no diff to speed) is that the W7 one dual boots with Linux - which I don't like very much, but maybe that's just me...