If you're talking about serving many requests cost-effectively, then really the problem is not the servers (except over-provisioning, which is rampant. Learn to use tools like AWS' auto-scaling system instead) - it's the code.

If you understand the basics of algorithmic time complexity (that's your Big-O notation) and profiling your code then you're ahead of 98% of other developers in practice. I'm constantly amazed at how many developers think adding more libraries, newer frameworks, or more layers of tooling will magically speed up their code because "it's so fast". If you actually time things you'll find out doing it the "slow" way is frequently an order of magnitude faster.