Hmm, you ask some questions with some very long answers, but in short, hardware speed and software speed are two different animals. You can have a very fast computer running a program that is written poorly and it will run equally slow on a super computer or a 10 year old Pentium 3. Windows 10 is optimized MUCH better than 7 ever was, and so the newer your hardware the better it will run.
That little thing about the fitbit, that is exactly why it's time to jump ship from win7, win7 is a dead OS, and more and more frequently you will run into the scenario where you cannot use xxxx piece of hardware unless you upgrade to win10. It isn't being updated by MS any longer, it isn't getting software written for it. It is obsolete, not to say it wasn't a great OS, it certainly was, but its old, its done, and its going to get harder and harder to avoid the win10 upgrade as time goes by.
Windows 7, at it's core, is based on a corporate level operating system released in 1998.
Windows 10, at it's core, is based on a consumer level operating system released in 2012.