EFB to RAM generates a texture for the gpu to use, and also copies the object to the gamecube/wii memory. This is required if a game reads this data and does *something* with it, use it for a screenshot feature, know what is on screen(scan visor Metroid Prime 2/3), modify textures(Last Story). Right now it's also required as a fallback for cases that aren't handled otherwise, like partial texture updates in new super mario bros for the spinning coins, the block animation, blue(or red?) coins, the question mark box animation etc. Those partial texture updates could be handled without efb2ram, but this would require some new code.
Anyways, to answer the question, both cpu and gpu are important for efb2ram performance. On this copy from efb to ram, the gpu has to finish rendering, before it can copy from gpu ram to cpu ram. During this the cpu is waiting for the gpu. And then afterwards the gpu is waiting for the cpu to give it new tasks. (fun fact: directly after the efb2ram copy, the cpu is also calculating the hash for the efb copy, so this wait period depends on the texture cache accuracy setting on some games.) So both cpu and gpu are not doing anything for while and have to do all the work in a shorter period of time than usual. This is the reason why efb2ram can cause massive slowdowns, and why a higher IR setting has a much bigger impact than it does on efb2tex.
PS: In the thread for the last story, i posted how to patch the dolphin to only do efb2ram when necessary in the game, this results in a good speedup. I don't know if it really works 100%, i only tested loading a game and recoloring clothes with it.
Anyways, to answer the question, both cpu and gpu are important for efb2ram performance. On this copy from efb to ram, the gpu has to finish rendering, before it can copy from gpu ram to cpu ram. During this the cpu is waiting for the gpu. And then afterwards the gpu is waiting for the cpu to give it new tasks. (fun fact: directly after the efb2ram copy, the cpu is also calculating the hash for the efb copy, so this wait period depends on the texture cache accuracy setting on some games.) So both cpu and gpu are not doing anything for while and have to do all the work in a shorter period of time than usual. This is the reason why efb2ram can cause massive slowdowns, and why a higher IR setting has a much bigger impact than it does on efb2tex.
PS: In the thread for the last story, i posted how to patch the dolphin to only do efb2ram when necessary in the game, this results in a good speedup. I don't know if it really works 100%, i only tested loading a game and recoloring clothes with it.