AWS Lambda reduced the billing granularity for Lambda function duration from 100ms down to 1ms. This will lower the price for most Lambda functions, more so for short duration functions. Their compute duration will be billed in 1ms increments instead of being rounded up to the nearest 100 ms increment per invoke. For example, a function that runs in 30ms on average used to be billed for 100ms. Now, it will be billed for 30ms resulting in a 70% drop in its duration spend.
Source:: Amazon AWS