watchOS2 animateWithDuration starts slow and speeds up
<p>I'm trying to animate the width of a group in my watchOS2 application by calling <code>animateWithDuration</code> of <code>WKInterfaceController</code> class. The idea is to show the user an horizontal line which decreases its width from right to left over a period of time (something like a timer):</p>

<pre><code>self.timer.setWidth(100)
self.animateWithDuration(NSTimeInterval(duration)) {
    self.timer.setWidth(0)
}
</code></pre>

<p>However I'm seeing that as soon as the animation starts the speed is very slow and then it increases. When the animation is about to stop (when the timer width is close to 0) the animation slows down again. 
I want the speed to be the same over the duration of the animation.</p>

<p>Has anyone had this issue before? Any help is appreciated! Thanks</p>