[wsh91]

There's a per-invocation cost that, at ridiculous volumes, becomes non-trivial ($0.20 per million). We get very high throughput (my back of the envelope math says about 3 writes per millisecond per Lambda, that's to a Cassandra cluster) for I/O intensive operations. You can have up to 100 simultaneous invocations, and you can ask for more (we did). Without knowing more about your situation, I would suggest that you use a library that lets you fire off a bunch of async requests and block on them all. Play around with RAM/CPU (one knob for both)--a higher setting may result in quicker processing at a lower cost (!). If you're highly cost sensitive, consider batching your SNS messages--remember that it supports 64K payloads. (We use SNS to do batchloading, actually--it's a cheap, managed alternative to Kinesis.)

Should you choose the fanout route, Tim Wagner from AWS told me that it's pretty fast: https://twitter.com/timallenwagner/status/658025794900365312