"GPU Process" Memory footprint
I noticed that after opening and closing/hibernating lots of tab, Vivaldi (or maybe Chromium engine?) is pretty bad in freeing memory.
After some tests, I found that ending "GPU Process" from Vivaldi's own task manager, resolves this issue to a high extend.
Since this process will restart automatically after it ending, and this really dose not affect the user session (your Vivaldi window will just become black for a second or two while the process restarts), I suggest developers to automate this process, if there is no easy resolution for that (and it seems there is not, since this an old issue, maybe the case is similar in Chrome, not sure).
So if Vivaldi, while idle, and say minimized, will periodically, or after heavy usage detection, restart this process, I believe it will reduce hugely memory footprint.