Why does increasing RAM decrease the lambda execution time?

Hey,

So I have come across this article that explains how increasing memory increases compute power. My code utilizes less than 2000 MB of RAM (1800 to be specific), however, the time to execute the code is around 8.3 seconds for 2GB of memory configured.

When I increase the memory to 4 GB the time reduces to 4 seconds. Now, I understand the concept that you pay for what you use. And for 4GB I am paying double than 2GB while I don't really need the RAM.

So the issue here is why is increasing RAM the only way to increase compute power?