The list is long: reflections and refractions increase rendering time significantly, anti-aliasing (0 longest)-BTW this you must edit in the .config file and resolve depth (also in .config file), number of polygons, camera position (obliqueness), size of brush maps and even the numerical entries of solid textures, resolution, display and render modes etc etc.
The big ones are refraction, edge level(antialiasing, reflection (along with "depth") and #of polygons. Pretty well in that order too. Remember that a higher refraction index is longer rendering time also. And yes the scale of the object means a LOT. Imagine uses something called an *Octree* to calculate the scene. This is related to the world size setting which is also discussed here in article number 1 above. The difference can go from *hours* to minutes, so scale your scene by the size you make your world. You can select everything in the scene (including camera and lights) and scale it interactively.