title: How can I "performance detect" browsers
body: <p>Some mobile browsers and IE8 javascript is not just a little bit slower, it's many times 10x slower! There are lots of things that pass feature detection tests (js css manipulations, etc) but are so slow they degrade the user experience.</p>
  
  <p>Modernizr will tell me that a feature exists, but it won't tell me if it's below some performance threshold.</p>
  
  <p>Detecting IE8 would work for IE8 users, but it won't work for slow mobile devices, tablets, and computers running old versions of FF, Safari, mobile Opera, etc.</p>
  
  <p>What are effective ways to temper or disable slow features without penalizing modern browser users?</p>
  
  <p>Are the better methods than time-stamping code execution blocks?</p>