It’s ironic that AWS touts the benefit Lambda gets from overcommit, but if you build a lambda that simply turns around and makes an api call, you are paying full price for the cpu usage, even though it’s idle.
It doesn't matter if it's more efficient for Amazon (which serverless very much is) if they don't pass on the savings to you. Lambda is priced as a "value add" not as an efficiency improvement.
You’re still consuming the RAM for the duration. In our on-prem VMWare environment, we didn’t charge, but we thought of the limited allocation being RAM far ahead of CPU ahead of disk.