Adding onto this thread that I am having the same issue. Downloads in Opera GX, latest version, browser VPN is Not turned on and Windows Defender is not causing the issue, downloads are insanely slow.
The internet speed test posted above from @10inchesunbuffed looks remarkably like the one I had when I tested my internet speed in Opera GX. In Firefox, my results are normal and what my ISP actually provides.
What's frustrating is that usually Opera GX can finish downloads under 1 - 2 MB, anything above however, it will download about halfway or so and just stop making any progress whatsoever on the download.
Needless to say, I've looked everywhere in my settings and I can only assume this is an issue with the browser itself, as I have tried everything. If there is some kind of optimization causing this, I want to turn it off. I would rather my browser work and be slightly slower, than be ultra fast and not work at all with arguably one of the most important aspects.
It's never a good thing when the solution is to boot up Firefox (an alternative to what is supposed to be my main browser) to the same download and it actually downloads (quicker) and finishes.
Another, unrelated, more minor complaint:
Booting the browser to the blank homepage takes longer than I would like it to. I see my background but the Google widget, alongside icons for my many bookmarks, menu functionality, etc. takes a moment after the browser itself opens.
A good solution would be to not open the main window until the browser has fully booted, or to optimize the way the browser boots and/or the handling of bookmark icon caching.