I have actually had pretty consistent fast start up times when launching Vivaldi. Really no different then Chrome or Firefox and perhaps even a little fast then the before mentioned. IE is not generally praised for much by many but is for its start up speed. Some factors to take into consideration is what homepage you have set as the default if you are questioning full start up times also and although there is not a lot of support yet for extensions, but what one's you may have installed. Have you watched your task manager and system resources tab when launching and running Vivaldi? I too will say Vivaldi is currently a tech preview, maybe not even an alpha, so a lot of tine tuning and system resource management stuff should be coming in the next releases along with improved and added functionality.
I like it already, although there is still a long way to go before it's ready to replace Opera 12.17 (my current default browser), and I already added it to my Open In menu. I added a page about Vivaldi to my Software Review site.
These are the main points I look forward to:
Tabs to top of window
Editable Mouse Gestures
More Keyboard Customisation: Ctrl W to close tab doesn't seem to work although I added it
Mnemonic Keyboard Shortcuts, e.g. S,B = Show status bar
Launch other Windows programs with "Execute Program" shortcuts