We fly our airplanes by looking at how they respond to the transmitter stick movement - thus you constantly adjust the amount of stick movement that you use to what you want the airplane to do. At higher speed all control surfaces become more sensitive to stick movement because of increased airflow over them ( that is why pylon racers have small control surfaces and use small amounts of control throw ) When pulling up at speed the plane will respond briskly to even the slightest stick movement and you automatically (sub-consciously ! ) down-tone the amount of stick movement that you use. When flying fast you may think that you "yanked" the stick up but that is unlikely. Really "yanking" the stick abruptly full back at high rates, at full throttle will most like not only cause a snap, but may even test the structural integrity of the whole airframe ! ! At slow speed the airplane response to stick movement becomes much more sluggish and automatically you will use much more stick movement. That is where the "yank" comes in that causes a slow speed snap. These differences in the amount of stick movement is easy to see when watching someone's hands when they fly at different speeds. There may be a slight uneven motion of the two elevators because of the control rod issue as you mentioned, but even if the movement was excactly the same it will still snap.