We're seeing some performance issues with MongoDB when there are a lot of indexes being built in the background. Currently, there is an index commit after every insert: https://github.com/mongodb/mongo/blob/master/src/mongo/db/catalog/index_create.cpp#L266

The performance is likely to be much better if we could batch a couple of writes together.

Is this something you would be interested in? I might be able to draft the patch, too.