We plan to set collect_metrics to false by default in C++ method searchKnn

[yurymalkov]

Right now it is true (https://github.com/nmslib/hnswlib/blob/master/hnswlib/hnswalg.h#L1220) and that is causing a measurable slowdown in multi-threaded search in some niche cases (e.g. dim=4).
collect_metrics increments some counters like number of distance computations or number of hops that can be used with C++ interface and is intended for debugging purposes. Not sure if anyone is actually using them.
We plan to change the default behavior to false in the next release.