Chromium eats ram like fire consumes oxygen, and Vivaldi is built on Chromium. Of course most users hover around a few specific sites, therefore don't often require more than a single tab or two, and don't typically notice the drain on their system. And who cares that your browser is using 2GB of ram with only two or three open tabs when computers are coming with 8GB of ram now'days? Most users hardly run anything but their browsers anyway, what could they possibly need that ram for?
But Vivaldi is marketed to "power users," some of whom are the type to more frequently open quite a lot more than 10 tabs. Vivaldi users are not Chrome users. These two demographics are fundamentally different due to Google's incessant push to dumb everything down until the software can only be used as the developers want it to be used, instead of how the user wants to use it.
I myself have seen Vivaldi use so much of my ram that the rest of my system slows down and ultimately becomes unresponsive, though I am on somewhat older hardware with only have 4gb of ram. Before I enabled the following flags, Vivaldi sat between 2 and 3gb on my system while doing nothing more than browsing Reddit. Now, it uses just over 1gb. These flags made Vivaldi noticeably more responsive and much lighter for me, but, obviously, mileage will vary depending on your hardware and operating system.
Before we get into the flags, extensions to automatically suspend or hibernate tabs when not used can be helpful, like The Great Suspender.
Enabling this allows GPU acceleration on unsupported systems by overriding Chromium's built-in software rendering.
Enabling this will allow Vivaldi to use opaque canvases.
This causes 2D canvas rasterization to be performed on a separate thread, thus causing processes to finish faster.
Enabling this will allow Vivaldi to switch between the various implementations of graphics rendering pipelines based on how the canvas is used in order to increase performance. For example, going from an implementation that uses the GPU to one that doesn't.
Enabling this allows Vivaldi to use the GPU to rasterize web content.
This will disable anti-aliasing.
Html videos have a tendency to eat system resources more heavily than Flash.
This permits servers to specify that some resources may be revalidated in the background to improve latency.
This will prevent Vivaldi from automatically reloading pages that failed to load while offline.
This will prevent Vivaldi from automatically reloading pages that failed to load while offline, even if it is in the active tab.
This forces Chromium to write directly to the GPU. This helps a lot with battery life. Read more about this flag, and about rasterization in general, here.
Higher values here will increase the number of images that can be loaded at the same time, so higher values means more resource usage and lower values means less.
This will allow Vivaldi to make use of the GPU while rendering content.
These are options that can be toggled in Vivaldi's settings to further improve performance and power management. Not all of them will be available to users of other Chromium browsers.