I typically open 14 tabs at once and webkit has been blindly fast. On this version I sometimes a) get the busy multicolor spinning ball, if I do the back button it sort of jerks, like it pauses and then goes back. I know that is subjective but it is very consistent and very different from other nightly builds ( 5525.20.1 is the one I'm using). I did add a 15th site, economist.com. It looks like a memory issue to me, but who know.