If you have an Nvidia GPU, you might have to goto the URL opera://flags/#use-angle, set the flag to "OpenGL" and restart. But, you can try the others if that doesn't help. Revert the flag back to default though if nothing helps
Also, goto the URL opera://settings/batterySaver and make sure the Battery Saver feature is off.
Are you talking about Discord in the sidebar or just in a regular tab? Any difference with the latter?