We all know that for a complex scene that has many and large objects, the fps becomes dramatically low (2 fps). This happens even after disabling the subdivision surface modifiers and hiding unnecessary objects and doing some optimization steps.
My question is: Which part of the hardware is responsible for the playback fps? Is it the Processor? RAM? Graphics Card?
both CPU and GPU, the CPU can bottleneck the GPU, and even with a good CPU capable of feeding the GPU, if the GPU is slow you will still be limited. However at a certain point it just comes down to the software. So hard to say if the HW is your problem or not.
How about you start by listing your current HW, and if you can, pick a test scene that we can all use as a control point. It could be one of the EEVEE example demo scenes from here:
I would pick the Tree Creature, since it features some animation too.
I don’t have answer to your questions but I recently discovered a bug report that says normal map node causes drastic performance issues on animation playback (and maybe in render as well). Try muting your normal map nodes and see if it helps. And check this: