Posting from Firefox while I wait for Vivaldi to wake up.
I don't have much to add other than my observation that this appears more as Vivaldi postponing things rather than sluggishness. I ask it to open a link, when it doesn't, I click on another tab to see if it will respond, do some other stuff it doesn't respond to and then a while it later it will simply respond to everything.
My problems also began with 6.2 if I am recalling correctly. At the moment they are quite reproducible. I have to blow Vivaldi away and restart four or five times a day every day.
I'm going to revert to 6.1. Before I do that is there anything I should do to make this a more productive diagnostic?