Windows 7 is pretty old, and many new browsers and websites might not work well on it. If your computer can handle it, moving to a newer Windows version could fix these problems.
Add-ons might make browsing slow if they're not well made or if you have too many open at once. To check if this is the problem in Opera, go to Extensions and then Manage Extensions. Turn them off one at a time to see if your browsing gets faster.