You know how Vivaldi is slow to create a new tab? Especially when you have a lot of tabs open.
This extension keeps a new tab already open waiting for you and switches to it if you want to create a new tab.

Chrome-based browsers prohibit overriding important shortcuts (as Ctrl/Cmd+T), so by default this extension uses Ctrl/Cmd+Shift+5 for this extension's "Create new tab or switch to the existing blank tab if any" shortcut. So if you want truly seamless experience, you'll need to have external system-wide app to convert Ctrl/Cmd+Shift+5 to Ctrl/Cmd+T.
I use BetterTouchTool on Mac for this and AutoHotKey on Windows.

Also, for some reason the shortcut doesn't work for me if it's in "Vivaldi-only" mode, so you might need to change it to Global at your "Extensions"->"Keyboard shortcuts" page. But Ctrl/Cmd+Shift+5 is pretty unused shortcut usually, so that doesn't bother me much. (You can also change the shortcut to something even more obscure on the same page.)