It's the sharpness, if you look in the original, there is torn geometry at the boundaries. This errored geometry will cause computational hangs. While TG is probably very optimized to quit these heavy areas before wasting too much time (hence crazy looking geometry/tears/holes/flipped through looking stuff), it still probably adds to the render time when it has to do it a million times.
Totally straight edges (on Y) aren't handled very well it seems with displacement [at least on built-in displaceable primitives] (and probably totally straight edges on any primary axis). So it's good to have a very faint, slight angle (fuzzy area).