My team at work has a neat feature that we've built on top of Lucene that has provided a substantial (20%+) increase in maximum qps and some reduction in query latency.
Basically, we run a training process that looks at historical queries to find frequently co-occurring combinations of required clauses, say "+A +B +C +D". Then at indexing time, if a document satisfies one of these known combinations, we add a new term to the doc, like "opto:ABCD". At query time, we can then replace the required clauses with a single TermQuery for the "optimized" term. It adds a little bit of extra work at indexing time and requires the offline training step, but we've found that it yields a significant boost at query time. We're interested in open-sourcing this feature. Is it something worth adding to Lucene? Since it doesn't require any core changes, maybe as a module?