How to compare the efficiency of public key cryptosystems, i.e., RSA vs El Gamal? As part of my Mathematics degree I'm taking a Discrete Mathematics module which partially covers Public Key Cryptography but does not at all enter it in depth.

I'm currently working on a project that involves writing about the process, security and efficiency (in terms of calculation/processing speed) of a few PKC systems. 

I understand the processes and how to discuss the security of these systems, but I'm struggling with what makes a PKC system "good" in terms of efficiency, at least comparatively. I'm only considering the following 3 systems: 


RSA, 
El Gamal, &amp; 
Knapsack.


On a basic level I can compare key sizes and ciphertext expansion, but further than that I'm a bit lost; especially in terms of terminology when attempting further reading. Any direction or advice would be greatly appreciated.
Since you are approaching this from the point of view of mathematics, I think the most fruitful avenue would be to look at asymptotic performance and security of the problems that the cryptosystems are based on.

For example, with RSA you need a modular exponentiation for every message you encrypt or sign. The time that takes depends on the length of the modulus as well as the exponent. The most efficient general purpose algorithm for breaking RSA is GNFS which also takes time dependent on the size of the modulus.

Find out the same for the other algorithms you are interested in and you can compare them and see e.g. whether one will asymptotically become more efficient at equal security.

This will allow you to steer clear of the various practical issues of how the different constants compare and what platform some performance measures might be relevant for.