Hi Zappa,
Yes, there is always too little time.
My suggestion to help that a little bit, keep asking questions. Besides that, set up small scenes and render privies in Stamp size. The primary stop in learning is a slow response from your "machine"; no matter the speed of your computer, it is always slow when it holds things that don't matter for that moment. I understand when little time is given, one like to have at least one thing rendered out as a reward.
A good mix is the best approach. Just a plane object and a sphere plus material and light, one light! Never toss too many things in a scene. A typical mistake is the illusion of progress by loading things in the scene because one has them. (… based on my observation over nearly two decades of mentoring artists)
Keep it super simple. Then explore while rendering in 1/4 of HD or smaller, just enough to get feedback. The more you learn, the faster you will be. No beefy CPU will replace this. 😉 (I have rendered as preview three projects this evening in 480x270 to get an idea of the speed and progress of a scene in terms of color and animation. That makes me faster than anyone who always renders as large as possible. I learned that in 2004 from an ILM artist.
The more you get familiar with the mix of parameters, the faster you get where you like to be, and so, the little time you have is better used. Yes, that is a little bit slower at first.
Your question, some materials can be reproduced quickly, but some need more complex calculations. This is all I can say, based on a general question.
Enjoy
