There are some improvements to search performance, for example:

• https://issues.apache.org/jira/browse/LUCENE-3653

  I've noticed that when doing thousands of searches in a single thread the average time is quite low i.e. a few milliseconds. When adding more concurrent searches doing exactly the same search the average time increases drastically.
  I've profiled the search classes and found that the whole of lucene blocks on

  org.apache.lucene.index.SegmentCoreReaders.getTermsReader
  org.apache.lucene.util.VirtualMethod
  public synchronized int getImplementationDistance
  org.apache.lucene.util.AttributeSourcew.getAttributeInterfaces

  These cause search times to increase from a few milliseconds to up to 2 seconds when doing 500 concurrent searches on the same in memory index. Note: That the index is not being updates at all, so not refresh methods are called at any stage.

As more concurrent searches occur, performance seems to degrade due to lucene synchronization. This effects the larger instances of JIRA.