are you running the speed test on the same computer that’s experiencing the issue? if you are I’d suggest trying to reset your network settings.

if the device is only a few months old, I’d personally check and see if you can still get AppleCare+ for it, and then pay whatever the deductible is and get it fixed. if that’s not an option it might not be worth paying whatever the full price is to have it repaired, in which case I’d suggest looking into getting a display and keyboard to use it as a ‘desktop’, as it’s a really capable machine

you say that it’s only happening in Opera GX, maybe you (or someone else) installed a “GX Mod”? I personally haven’t used GX in a while but I’d bet you could find the GX Mod settings if you search in the browser settings.