I know Firefox was terribly slow there (like it is with many script-driven websites), but I never dug into why.

There's 3 separate engines all with different tradeoffs.

Have you used Firefox in the past couple of years?

Even if it were slow (which it isn't), it's not doing surveillance for money. If you really want a Chromium-based browser, you could always use Vivaldi or Opera or take your pick of many others.

Glacially slow.

And the recent buzz about a concurrent engine shows how much slower. When you have 300%+ headroom just by uncoupling your execution strategy from an unnecessary global lock no one else has, you are not "speeding up". You are "catching up."

Benchmarks suggest there isn't much in it....

http://hothardware.com/reviews/web-browser-performance-compa...

Firefox is sluggish on every machine, and consistently feels like the slower paint. I understand the synthetic benchmarks are not so distant, but we don't interact with browsers at the level of Sunspider benchmarks. We interact at the level of full dom renders to screen, first clicks available, scrolling and repainting performance, tab open and close time, and startup time.

Firefox has never been a positive experience for me on those on anything but the very oldest of Linux boxes, and quite frankly I've written off the entire X11/Linux desktop environment as a tire fire with people trapped inside.

Also, note the 12 second time is what the Google+ team clocked as average. Yours may have been faster. Others, were even slower.