For clarification, Lambda Layers is the packaging of a node_module, vendor/gem, or python package folder, and then "sharing" that across various Lambdas, instead of deploying the shared files again and again.

The actual lambda file ends up being a few hundred bytes, because they are literally just one function in a single file.

There is no compiling involved in the Layers after its made once. The only changing part of the Lambda is the lambda_handler.

My understanding is that a custom lambda image is the nuclear option; having everything fit cleanly into a normal lambda archive is the preferred option; and lambda layers are a type of middle ground if you've outgrown one but don't quite need the other.