Background: I just installed Vivaldi yesterday as an alternative to a secondary browser that broke in its migration to QT6.
In looking for a way to suppress the display of thumbnails, I came across this feature request which is now 6 years old.
I saw the recommendation to use CSS to suppress the display, but as others have noted, this does not suppress the creation of thumbnails and the resulting bloat.
While I'm new to Vivaldi, I started programming in 1978. I went from teletype time sharing to line printers to keypunch to video terminals to personal computers. I started with Slackware in the '90s and currently use ArchLinux as my distribution of choice. All of which is to say that I'm not averse to manually editing CSS or configuration files.
In fact, I'm considering writing a script to periodically strip thumbnails from the configuration, both embedded Base64 images as well as the JPGs under "Vivaldi Thumbnails".
However, in my initial investigation, I discovered even more pressing reasons to request the Vivaldi team to disable this "feature" by default: Privacy and Security.
Purging browser history would not remove thumbnails for bookmarked sites. Imaging my surprise when an updated thumbnail for Duck Duck Go included a recent search and its results. I know for a fact that the initial thumbnail was an empty search page. So the user has no control whatsoever of what data might be thumbnailed.
While there is a performance benefit to caching page elements, that is not true of these thumbnails - quite the opposite. Thumbnail creation adversely impacts performance and data footprint (as previously documented earlier in this thread).
I'll spend some time seeing what I can do to mitigate this issue on my end, but if it's too cumbersome, this power user will be looking for another alternative, even if I have to compile it from the AUR.
But frankly, this should have been addressed long ago. At the very least, thumbnails should not be embedded in the bookmarks file to address the bloat. Just a pointer to the cached image would do. This would also allow for easier management if all the bookmark thumbnails were cached under a single directory. (It should also be an easier fix than adding a configuration option to change browser behavior.)