Lambda function code tends to be fairly single threaded, as others have mentioned, using a lower memory size with Lambda, also lowers the amount of CPU performance available.

However, it appears to not only lower the amount of cores available, but the performance of those cores.

So even if your code is very single threaded, and has low memory requirements, you still might want to provision a larger lambda memory size.

The frustrating thing with this, is that your single threaded code might only be using one of up to 6 CPU cores made available to it.