Common problems and possible reasons/solutions:
Corporate laptops: While used by another member of the family "something happened" and now your laptop is very slow.
Look for unusual or new icons in your system tray (in Windows box at the bottom right with your clock in it). The icons are applications running on your system One of them may be downloading something, sharing something or otherwise making your laptop and/or Internet connection slower than you're used to. Try right-clicking the icon to see if there is an EXIT option. If that fixes your issue, you'll need to uninstall the application if you don't want it to start again after the next reboot.