7 and 10 run about the same for me on low-powered computers. Win 10 is initially very slow with a platter drive, but it speeds up once Superfetch and things cache data commonly used on startup. Win 7 has a bagillion updates to apply, so that will take some initial time to apply.
If said computer has 2 GB of RAM, I wouldn't even bother with 10. If it has 4 GB, I consider that the bare minimum to run 7 or 10 these days for basic computer tasks (web browsing, office productivity) without driving me crazy.