[saurik]

An NSRunLoop is normally a CFRunLoop, and a CFRunLoop is pretty much just "get next task; run that task; loop": if one task blocks, it cannot run the next one; the only way for it to look like it is doing multiple things at once is for code to reenter the runloop using CFRunLoopRun(), which is hardly ever used (it increases the size of the stack pretty quickly and breaks assumptions about single-threaded concurrency). Any concurrency with hardware compositing is going to happen at the kernel or hardware levels, not in the app.