title: sbt assembly task runs slowly after adding some dependencies
body: <p>I'm kind of new to deployment in scala and I configured the <code>sbt-assembly</code> plugin, all worked well.</p>
  
  <p>Some days ago I added hadoop, spark and some other dependencies, then the <code>assembly</code> task become extremely slow (8 to 10 minutes) and before that, it was &lt; 30s. <strong>Most of the time is used for generating the assembly-jar (it takes several seconds for the jar to grow 1MB in size).</strong></p>
  
  <p>I observed that there's a lot of merge conflicts, which are resolved by <code>first</code> strategy. Does this affects the speed of assembly? </p>
  
  <p>I've played with the -Xmx option for sbt (add -Xmx4096m) but it doesn't help.</p>
  
  <p>I'm using <code>sbt</code> 12.4 and <code>sbt-assembly</code>. Any suggestions or pointers for optimize this task?</p>