[Manishearth]

Chrome is one process per tab (well, per origin, because same-origin JS), and within a tab is mostly single threaded (at least, this is my understanding).

Servo has a one process per tab mode (off by default), but regardless of that within a single tab both styling and layout can be parallelized across many threads, and rendering makes full use of the GPU.

This means that the active tab will be much faster. You don't care about rendering/layout speed for inactive tabs, and usually not styling speed either (restyles usually get triggered by interaction, though if you have a CSS game or something in a background tab that might matter). So process-per-tab doesn't make use of the resources it could, unless you're parallelizing within the tab. I suspect most background tab processes will be asleep or doing much less work; so while it helps for isolation (security, and less jank caused by one badly performing tab), it doesn't help as much for response time in a single tab.