Is performance tuning a relevant topic for AWS Lambda though?
Lambdas are also used for request/response workflows, not necessarily just async background tasks.
Another use case is lambda resolvers with AppSync if you need some sort of data that can't be obtained with the native resolvers (i.e. DynamoDB)
Beyond the choice of the lambda runtime and how much RAM is provisioned, what else is there that's worth being tuned?
The amount of RAM being allocated to a lambda function also controls the vCPU granted to the lambda function.
There's a nice balance that can be struck with power tuning where you're paying more per millisecond for a higher RAM configuration but the duration of each invocation improves enough to the point where you're actually paying less.