A flyline loop is a projectile. Fast rods can cast tighter loops, which are more streamlined projectiles. In bullets, the difference in bullet shapes: pointed vs. round- or flat nose, are quantified as ballistic coefficients. So tight loops go farther, faster. The difference isn't huge, perhaps 5-10 per cent. And for fly casters who don't know how to cast tight loops using their skills, a faster rod is no advantage.