I used Vivaldi for a couple of months. Plenty of goodness, and one major showstopper: it's slow. It's so very sluggish and slow. Something as trivial as tearing a tab off a window would take several seconds. Switching between tabs was laggy as well, once there were many. Every other browser felt blazingly fast after that. That's a pity since this type of issues usually indicates some deeper design problems that won't be easily fixable.