So, in two words : nobody knows ? ;)
How fast is MUI, compared with vanilla Intuition ? chess GUIs are very speed-critical: they must take as less CPU time as possible to leave all of it to the engines. Engines send kilobytes of informations back to the GUI per second : current move examined, how many nodes traveled through, time used, best line, etc... The GUI received those informations, parse them and use them accordingly, like for instance by displaying the current best variation to the user, etc... So it has to use the fastest functions the OS can provide.
Maybe I'm wrong, but it is my understanding that MUI sits on top of Intuition, and so adds a layer to the OS and must then be a bit slower than vanilla Intuition.
But my problem, really, isn't the GUI itself, it's communicating with a cli-program : sending it commands and receiving its output back, while keeping the communication canal opened throughout the whole game.
"A good bug is a dead bug" (Don Dailey)