Yes, the performance will matter. But as Servo demonstrates there are a lot of speed gains to be made with the right design.
See this talk by Patrick Walton about Servo's WebRender: https://air.mozilla.org/bay-area-rust-meetup-february-2016/#...
And this short demo comparing Chrome, Firefox, Safari, and Servo: https://www.youtube.com/watch?v=u0hYIRQRiws