Kill Opera in the task manager, restart your computer, uninstall Opera (don't choose to delete your data for now), and reinstall.
If that doesn't do the trick, test in a standalone installation. If Opera works fine there, you know it's something with your profile. If Opera doesn't work fine there either, goto the URL opera://settings/system and disable hardware acceleration. When you kill Opera and start it back up, goto the URL again and make sure hardware acceleration is still off. If not, kill Opera and modify its shortcut to pass --disable-gpu to launcher.exe. If that doesn't work, I'd look for issues outside of Opera.
Now, if you find out it's your Opera profile that's causing the issue, I would goto the URL opera://about, take note of the "profile" path, kill Opera, and delete the 3 history files in Opera's profile directory. You'll lose your history of course, but that might fix it as those files recently got corrupted for some users.
For anyone in 2022 June still experiencing slow loading times, lagging and stuttering when opening new tabs and page rendering in Opera, try tinkering with opera://flags/#use-angle setting. Switching it from the default to OpenGL did the trick.