title: Xcode 10 iOS app build, "hidden" task takes most of build time
body: <p>After changing single line of code and performing Xcode 10.1 incremental build in large project, Xcode spends most of build time in "Compile swift source files" phase after completing all listed tasks (compile changed files and merge swiftmodule are both completed)
  screenshot showing tasks list: <a href="https://imgur.com/a/JoVI0zB" rel="noreferrer">https://imgur.com/a/JoVI0zB</a></p>
  
  <p>While compilation and merging swift module takes less than second, whole phase can take up to 2min in my project (300k LOC).</p>
  
  <p>What does Xcode do in this time? Is there any way to speed up this process?</p>
  
  <p>Similar project written in Obj-C takes just few seconds to launch after changing 1 line of code.</p>