The render process in general.
I have seen a lot of images and animations where the rendertimes a very low, and im always wondering how they do that without a lot of noise.
What the specific settings do, when to change them and to what.
I remember i once saw a Blender tutorial, where when you changed the tile size, rendering became way faster.... just small tricks

Win 10 - 64 bit, Intel Corei7-7800X CPU @ 3.50GHZ, 32 gb ram, 2x Gigabyte RTX 2060 SUPER 8GB, OctaneRender for Blender - The most recent stable build - Houdini indie 18.5.499