I haven't followed (and probably don't know nearly enough anyway) details of e10s, but their claim is contrary to your view:
> Performance would improve because the browser UI would not be affected by poor performance of content code (be it layout or JavaScript). Also, content processes could be isolated from each other, which would have similar security and performance benefits.
> Performance would improve because the browser UI would not be affected by poor performance of content code (be it layout or JavaScript). Also, content processes could be isolated from each other, which would have similar security and performance benefits.
https://wiki.mozilla.org/Electrolysis#Goal