BSONArrayBuilder::fill is declared as follows:

void fill( const StringData& name )

{ char *r; int n = strtol( name.data(), &r, 10 ); uassert( 13048, (string)"can't append to array using string field name [" + name.data() + "]" , !*r ); while( _i < n ) append( nullElt() ); }

That 'uassert' call ends up constructing a temporary string, appending to it, and then passing it by value to uassert, all to determine that r is not NULL in the vast majority of cases. From what i can tell in bson.h, the uassert call is not conditional on a debug build, or something similar.

Removing the uassert dramatically improves the performance of adding elements to a BSON array.