Did the laptop get ‘noticeably slower’ since the install of win10? If yes, and because it’s known that 10 uses more resources, that’s the issue.
If no, then I’d bet on malware. I helped a neighbor with a slow laptop, we installed the free version of malware-bytes and found over a thousand instances of malware.
The cpu was using about 25% of resources with nothing open! I’d suggest to always scan for malware regularly, no matters what.