Mindy,

When you "render" you're asking UF to regenerate the entire image at high-res. The resulting image is automatically flattened as it's generated, unless you asked for a .PSD file, so rendering in UF doesn't require extreme amounts of memory even if you have a bazillion layers. However, UF still has to generate all the pixels for all the layers, so that's an awful lot of computation.

Remember that in UF, an image isn't just a collection of pixels; it's a collection of mathematical parameters, which are on-the-fly rendered on-screen as you work. It's like you're interactively building a set of instructions of how to make your image, and you can go back to any of the instructions and change them, and UF will rebuild your image from scratch. It is not like PS, where you proceed step-by-step, each modifying the previous step. In PS, when you save your image, you're saving the final result of all your pixel-pushing. You're not saving the step-by-step process(*). When you open parameters in UF, it's like you're going through the command history and reassembling your whole image, because that's UF's native operation. UF never manipulates pixel data directly; it's always manipulated via formula.

So no, you can't "flatten before you render" because that's something you do to pixels, not fractal formulas. UF just flattens-as-it-goes, whenever it's creating pixel data, for screen or final render.

(*) Yes, you can save your command history and use it to reconstruct your image from scratch, but that's not the primary workflow, and resizing your image doesn't automatically translate all your steps.

--Damien