I find it strange that there is no discussion of the performance of writing data. Cassandra and Mongo differ hugely in this respect, and I expect you will see

I find it strange that there is no discussion of the performance of writing data. Cassandra and Mongo differ hugely in this respect, and I expect you will see huge performance gains in write performance. Mongo's write locking will mean that reads will be blocked while you are inserting data. Reads in Cassandra may trigger compaction and or require sequential IO if the table has not yet been compacted, so the tradeoff is interesting