"Performance" in the context of e10s means improving user responsiveness rather than throughput. For instance, you can usually smoothly scroll a page or interact with the browser UI even if the page is some kind of heavyweight mess.
Why couldn't that be achieved with threads? You can have asynchronous threading, I believe. (I get the arguments in favor of e10s, like sandboxing, and I have no beef with e10s, I'm just curious.)
