It's not just X. Most large systems involve one process doing work on behalf of another. (The elephant in the room, as ever, is database servers, which often have similar problems internally as well as between them and their client processes.)
Older Unixes often had *severe* X interactivity problems. X has long been famous for its jerky rendering (especially `jerky mouse syndrome') and CPU hogginess; the jerky mouse has gone away of late, but only by the cheat of running the mouse pointer at insanely high priority and because machines are now fast enough that moving a mouse pointer is cheap.
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