I guess it's because PerfTest is having a dominant bottleneck which isn't indexing: if you disable indexing using the blackhole backend, its performance increases only marginally. If you where to measure indexing only, it necessarily _at least_ doubles performance as it literally needs to split each update in two different operations which are approximately of same cost. On top of that, when using NRT an ADD operation is buffered but a DELETE requires a synchronous flush to the Directory. Normally an UPDATE operation is decomposed in the backend as a DELETE+ADD (it does it only at the end to make sure it applies both atomically), this flag allows to send just the ADD.. so yes 2X _at least_ is the improvement on indexing, way way more in NRT as otherwise the buffering opportunity is very unlikely.

It definitely requires a more complex test, what I'm explaining here is mostly theory in the scope of this pull, but backed by tests in HSearch. For example, the Search Engine is able to postpone the synch io needed on deletes until a query is actually run.. so that additional cost is "hidden" if you're not running any query at all.