Emre Kıcıman and Benjamin Livshits from Microsoft Research present some interesting data in their SOPS paper - AjaxScope: A Platform for Remotely Monitoring the Client-Side Behavior of Web 2.0 Applications.
What I want to highlight, though, are some of the measurements they present for both IE (6 and 7) as well as Firefox (2). This is real data in a refereed journal of the ACM, not a Gartner-style whitepaper.
Among the interesting nuggets are IE's 35x slower performance in String cat operations, and Firefox's 4x slower Array join execution time. The authors also put the intrinsics into context by measuring the performance of common portal pages - IE beats Firefox on msn.com, but Firefox turns the tables on Yahoo!.
Lots more interesting data, and a useful tool, in the paper. Read it.