Well, I'd be happy to swap issues with you, because I DO have the "delay loading of background tabs" selected, but they all load, which is NOT what I want at all, as it slows down opening & closing Opera, and runs up my RAM & CPU Usage big time! Maybe the key is to select the opposite of what we want in the preferences, maybe if I not select it, the background tabs won't open, and vice versa. But it is quite odd that we are both experiencing Opera not doing what we've selected, really odd...
Cmd + Shft + V reminds me of Ctrl + Shift + V, the latter keyboard shortcut assigns Opera on Windows by default to the command "Paste and go", thus also here the usual keyboard command for pasting (Ctrl+V), combined with a second control key; and Opera jumps to the address bar before inserting the URL and going to it.
So could there be a conflict with the default advanced shortcuts?
That's just a shot in the dark, I apologize if I'm wrong with my thinking.
I've seen the leak three or four times more yesterday, while in Opera Beta or Dev. Apparently not related to the content of the page, but I might be wrong here. Before that I thought it had to do with Discourse, but it happened also on other pages, even with only "opera:sync-internals" open.
But it is definitely not related to Gmail.
Anyway, I don't care much anymore. In the meantime Opera exposed so many other technical problems and design problems that I moved on to Vivaldi. This thing is felt 50× better than Opera and might be a worthy candidate for my future default browser. (Well, currently it already is my default browser, but still on a trial level.)
Nevertheless many thanks for your replies and support efforts, @leocg
The problem is most likely the file size. I have some 800 bookmarks but no favicons, so the file size for the .html file is less than 200 kb. There is a way to remove favicons from a bookmarks.html file. See this page and look at step 2.