title: A multimap with good performance
body: <p>In my code I have a map that is used heavily, several thousand times in a few seconds. Originally I had a TreeMap, but when testing with 9,000 entries I watched my old processor melt. And this needs to scale. So I moved to a HashMap and performance was excellent.</p>
  
  <p>Now I am changing my design and am looking for a MultiMap. However I'm afraid of the performance impact on the <code>get()</code> side, as it must iterate over said large map picking out the matching keys, and when called many many times even synchronized it seems like it would be slow.</p>
  
  <p>Is there a good MultiMap that can handle such large values with great performance? Performance is critical in this application, as there could be many large separate maps handling a very large workload, making "small" performance losses very big issues.</p>
  
  <p>Bonus points if it can be extracted to work alone without any dependencies. </p>