Since Microsoft only compares using the outdated Sunspider benchmark, I decided to find out how IE compares using modern, proper benchmarks. Here are the results:

Google Octane: https://developers.google.com/octane/

(Higher is better)

Google Chrome 30.0.1599.101: 11337

Mozilla Firefox 25.0: 10127

Internet Explorer 11: 7631

Mozilla Kraken: http://krakenbenchmark.mozilla.org/ (Lower is better)

Google Chrome 30.0.1599.101: 2479.4ms

Mozilla Firefox 25.0: 2653.1ms

Internet Explorer 11: 4792.3ms

As you can see, IE11 lags pretty far behind both Google Chrome and Mozilla Firefox in modern JavaScript benchmarks. So, it would appear that IE is specifically geared towards the outdated Sunspider benchmark. That optimization coupled with the poor performance on Octane and Kraken is likely why Microsoft only ever mentions Sunspider.

Testing Notes: The above test was performed on a fully-patched (as of 2013-11-07) Windows 7 Ultimate x64 machine with antivirus and all other background apps disabled. Each browser was started fresh for each test and had no extensions/add-ons enabled. Hardware-wise, the machine is an Intel Core2 Quad Q6600 running at 2.4GHz with 8GB of PC2-6400 DDR2 RAM clocked at 400MHz (x2 = 800MHz) and an AMD Radeon HD 6700 running at 850MHz with 1GB of RAM @ 1200MHz running Catalyst 13.9.