Parallel reductions are more heavy-weight synchronizations than locks. Say we have 64 partitions, then we need to perform 6 levels of tree reduction, or avoid parallelism completely and perform the reduction on a single thread. Either way it was slower.

The locking strategy very rarely had any reduction in parallelism due to the randomized lock-taking.

There were also other reasons, such as not wanting to replicate the filter per-thread.