Welcome to WebmasterWorld Guest from 18.104.22.168
For the minute the only way I test the efficiency of a script is by using a time function to see how long it takes to load (obviously the quicker the better).
Do we think about system resources? Definitely. How do we evaluate their usage? Speed under load. If you get 10 users on the same page and it starts lagging, big trouble. We all want a million uniques after all so that page better render blazingly fast.
The key to the whole thing is, the quicker the page renders the faster system resources are released. So really, it is all about speed.
<added>working on this post and Asandir answers, so I made some changes