To continue the last post about GENPlusDroid real time rewind. NESDroid rewind uses the exact same method. Required only some small changes to the emulation core, nothing major just how it loads/saves its state. The FCEU state saves are around 75Kb per frame. We are using a 20MB heap for the NES version which only provides 15-20s of rewind as you see in the final segment of the video.
Certainly more fun to real time rewind NES that Genesis in my honest opinion.