I have definitely noticed this issue. It seems to be more likely to freeze up if the feature line is long, with curves, and/or has tons of vertices. If its short and simple, breaking doesn't seem to be an issue.
I have experimented with exploding the feature line into a 3D poly, using the conventional BREAK command (which doesn't freeze Civil 3D), then converting it back to a feature line to benefit from the cool editing tools.