There's a warmup time when a function is first executed in a container that can be up to 2 seconds depending on what libraries you include (you pay for this time [1]). After the first execution, your function environment will stay "warm" for 10-15 minutes and it will only take 10-150ms to execute your function on a new event.
The other big downside is every time your function goes "cold" you pay the cost of all the extra (anything not in standard lib, ImageMagick, or the AWS Nodejs SDK) libraries you need.
The upside is that you only pay for the time your code actually runs. I've replaced a cronjob server and saved ~90% on the bill to run my jobs. Mostly they were scheduled backups and other misc integrity checks on AWS and 3rd-party infra, so nothing that intense.
1: the cost for 2 seconds of execution time on a 512-MB execution environment is 0.00001668 USD, so it's not likely to break the bank. If you have a high-traffic function it's likely to stay "warm" pretty much all the time. And if your function is low-traffic, it's likely you fit inside the free tier.
Ha! Cronjob to keep functions from going cold! If everyone starts doing that, it will reck the cost structure of Lambda for Amazon, I wager. One wonders why they didn't consider the possibility of users doing just that.
