title: Why does the first run of "XCTestCase -measureBlock:" takes so much time?
body: <p>I'm using Xcode 7. iPhone 5s simulator (iOS 9.0).</p>
  
  <p>I'm trying to use <code>XCTestCase -measureBlock:</code> to measure time spent on some of my functions. This runs the code inside the block 10 times and report the results how long each run takes, the average, STDEV, etc.</p>
  
  <p>It turns out that the time of <strong>first run is always very high, like 400-500% higher.</strong> Same happens for a very simple method, or even nothing in the measure block. I don't have anything in my <code>- (void)setUp</code> or <code>- (void)tearDown</code>.</p>
  
  <p>Result of <code>NSString -stringWithFormat:</code>. See the spike at the first bar of the graph.
  <a href="https://i.stack.imgur.com/PCGIW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PCGIW.png" alt="enter image description here"></a>
  <code>values: [0.000031, 0.000005, 0.000003, 0.000003, 0.000003, 0.000003, 0.000003, 0.000003, 0.000003, 0.000003]</code></p>
  
  <p>Result of a blank block. Similar results.
  <a href="https://i.stack.imgur.com/m7i7E.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/m7i7E.png" alt="enter image description here"></a>
  <code>values: [0.000007, 0.000001, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000001]</code></p>
  
  <p>Is this a bug or something? What is going on?</p>