Best method to move result of a query to another collection in mongodb

Question

I need to move the result of a query from a master collection to a slave collection. Currently I have all the IDs to move in a list records_to_move and there will be 300000 - 1000000 records. I am using pymongo.

for record in db.master.find({'_id': {'$in': records_to_move}}):
    db.slave.insert(record)
db.master.remove({'_id': {'$in': records_to_move}})

This is taking a lot of time since it is a exhaustive way. Is there a better way to do this?