I used a really low-end system for a while some time back, running Linux, and WebKit-based browsers were the only ones with a mainstream (so: actually renders correctly for practically all sites) engine that was usable with even one tab open (I could do 2-3 as long as none of the pages were “webapps”)

This indicates some kind of fundamentally better design, to me. Probably related to why Safari’s by far the most respectful to battery life, of the big three browsers.