There are a couple of typically problematic patterns that could indicate a potential performance bottleneck:

• Repeated calls to BSONObj::getField (e.g. we want to retrieve multiple fields in a loop) rather than an up-front call to getFields or iterating over all fields in the BSONObj in one go rather than doing a linear search for each field)
• Uses of BSON macros or BSONObjBuilder outside of testing/parsing code

We should see if we can spot any of these & fix them- and consider introducing some kind of linting rule or automation to try to prevent new instances of these patterns emerging.