I have not used windows for as long as I can remember, but I am sure you can test the directx vs opengl backends. The developers say directx is faster but less accurate than opengl. On my low end system (intel gpu, 2x intel @ 2.2ghz) dolphin runs most games quickly enough to be playable.
As for the dependencies. How much are truly optional I dunno but I know most of the sound backends are. I usually only build the alsa and pulse backends. Though statically I believe the dolphin source has stuff like wxwidgets in the source so if it does not find it on your system it will use the internal devel stuff to build. I used wx as an example because dolphin depends on a devel version of wx that is not in the main repositories of most distros. Rather than find it I just build it statically off the dolphin source and it works fine. Generally unless there are problems you should want to use the systems version of the libs and not the internal ones.