I am doing a small animation and was wondering which materials to avoid ? Which ones take longer to render?
Well, it depends hugely on material settings most of the time.
But I'd avoid Translucent, Anisotropic, Velvet, Gem, Leather & Metallic Paint to name a few. Also avoid high sample numbers and usually a number of 3-5 Ray Bounces is very sufficient. Use a Diffuse material wherever you can, it takes the less time in general.