Image-based lighting (HDRI) is usually a time saver (as I stated above).
Depending on the scene setup. With reflections ... not so much. Compared to a single spotlight, it's slower. Used as background, I agree; only you need quiet a bit of resolution, if the background should be sharp. Actually hdris are then a time saver when they are smaller and blurred (not so much to compute anymore).
There is a reason that something like SIBL was invented more than a decade ago (not available for Cheetah, but could actually easily be done), where you have a small, blurred hdr for lighting and high resolution ones as reflection maps (or even jpegs). The highest resolution is a jpeg for the background (but as only a fraction of it is shown in the actual render window, the available SIBL pictures have not a resolution big enough to work for todays high resolution renders (in this case, though, it could work). Stuff like this is faster, and that's why the workflow is incorporated in todays 3d apps, where you can control this stuff. It works only really well when the different pics work together. Something that Cheetah does not provide at the moment (it can be simulated with spheres).
That said, in my opinion HDRIs are more or less the only way to get decent lighting in Cheetah (often combined with other light sources). But how fast it is depends very much on the HDR used (I often use very high resolution where the hdr alone eats a giga of RAM).
so using things like the metal and dielectric shaders when you can, can be a major time saver.
??? I thought they're even slower than the traditional materials to recreate glass and metal (but deliver far better quality). It would be faster to try
not to use materials like metal and glass at all (or as few such materials as possible).
> High quality textures
They are not needed here. Some 2k is in this scenario more then enough (and for models they can be much smaller, very small if repeated).
Again, big pics can eat a lot of RAM.
All this stuff is very dependend of the scene. For example, images as backgrounds (and they don't have to be hdris when they don't contribute to the lighting) don't work with changed camera angles (or with the changed lighting).