In a performance investigation of initial sync, we found that for certain workloads, there is a bottleneck in performing BSON validation. By turning off validation on the source and target, initial sync time was reduced by up to 18%. The greatest savings occurred with large documents (since the bottleneck for small documents is insertion on the target) and documents containing nested arrays (since BSON validation was slower in this case).

We should investigate if there is any low-hanging fruit in improving performance of BSON validation.