We are using Node.js. From what I’ve seen online, Python should be the fastest though.
Edit: wanted to add, that from what I’ve gathered from people testing online, bundle size didn’t really matter, but perhaps someone else has some information that points to the contrary?
Bundle size is rarely equivalent to size of initial executable code. i.e. I can have a giant photo of my dog in the bundle but it won't necessarily affect the start up time of the node index.js. I think there is some kind of effect in that the bundle must be downloaded to the Lambda container server from S3, but that seems pretty fast, and it's likely cached there for a while.
Edit: wanted to add, that from what I’ve gathered from people testing online, bundle size didn’t really matter, but perhaps someone else has some information that points to the contrary?