- I don't see any mongodb index creation, so mongodb is inserting with no index while rethinkdb is inserting with the index. That's probably why there's a gap between the two

- there's no mongodb index, and rethinkdb queries do not make use of the index (this is probably why rethinkdb is not 1000x slower: both aren't using indexes)

- the $in query should be last_update: random_timestamp(), there's no need for $in here

- his insertion code creates 100K memory clones of the object to insert in the mongodb version only, not in rethinkdb

I'm sad to add: what the author is benchmarking here is the likely performance of a system he could build with either db. It's not necessarily bad (save for bad press) that he's bad at benchmarking: the mistakes he's made in his benchmark are similar to the mistakes he'll make in his code.

But yes, the author may use some help!