I'm about as far west as one can get, in the lower 48 -- near Seattle, WA. It's possible we have different mirrors, but I got the same results from the Major Geeks site as well. It seems the .ISO file is simply flawed.
Vivaldi and Firefox are very different. Our locations and ISPs are very different. About the only commonality is the .ISO file that KDE created and made available. That said, it might be possible to download one of the other ones, so although it's not what I really want, I think I'll try to download one of those, just for comparison.
...
No, the "testing" .ISO behaves just the same.
Then this, after a couple more minutes with nothing more happening.
That pretty much says it all. KDE is having network infrastructure issues that are making it impossible to download an .ISO from them. I even suspect that the Major Geeks site simply redirects the download request to KDE. Until tomorrow ...