When updating core for generics, I found the following as a optimization of FieldDocSortedHitQueue:
All FieldDoc values are Compareables (also the score or docid, if they
appear as SortField in a MultiSearcher or ParallelMultiSearcher). The code
of lessThan seems very ineffective, as it has a big switch statement on the
SortField type, then casts the value to the underlying numeric type Object,
calls Number.xxxValue() & co for it and then compares manually. As
j.l.Number is itself Comparable, I see no reason to do this. Just call
compareTo on the Comparable interface and we are happy. The big deal is that
it prevents casting and the two method calls xxxValue(), as Number.compareTo
works more efficient internally.
The only special cases are String sort, where the Locale may be used and the
score sorting which is backwards. But these are two if statements instead of
the whole switch.
I had not tested it now for performance, but in my opinion it should be
faster for MultiSearchers. All tests still pass (because they should).