Bson-ext slower than bson?

From the readme:

This module is a C++ addon for Node.js that implements the BSON parsing and serialization outside of the JavaScript environment in order to increase speed and memory efficiency for certain workloads.

Do we know under which workloads specifically bson-ext is faster than bson? I’ve ran it with a fairly trivial case (strings, numbers, dates) - and it was faster, but once I start nesting objects, it becomes much slower (approximately 30%)

Is this expected?