What to do if the internet is slow on one device?

It's probably not really an internet problem, sounds more like something with the laptop itself. I had something similar happen - turned out a ton of temp files had piled up and the browser was crawling like a snail, even though WiFi was working fine. Try clearing your browser cache and system temp files, check how much free disk space you've got, and make sure you don't have like a hundred tabs open at once. I'd also recommend reinstalling your network card drivers - sometimes after a Windows update they start acting weird, and you've got internet technically but everything loads like you're on the moon.