I don't know about Windows 11, but Windows 10 is still very snappy on old hardware with low ram. Windows has a lot of issues but desktop responsiveness isn't typically one of them.
No ads on my windows 10. Yes you have to configure it a bit after install, but once you do it can be pretty lean. I ran it on a machine with no more than 4gb for a few years and it was fine, clean and fast.