For fly casting our fly to the fish we should match line speed as good as possible. But in fishing reality most of us use a significant extra line speed for example to compensate typical casting issues. These issues are mostly about not matching trajectory, bad positioning of the rotation within the casting stroke, too much slack line, bad timing, not matching the size of casting arc and stroke, not keeping the rod in plane and simply shaping improper loops.
The extra line speed can also be used to fight wind or trying to reach a distance, which yet we don't constantly master to hit.
It's fair to summarize that most fly fishermen often use that significant extra in line speed all day long, because they are used to it! Believe me, it's so easy to get used to it. That happens in no time. Much easier to get used to a significant extra line speed as to improve one's fly casting and get used to just produce the level of line speed really being necessary to gently bring the fly to the target. BUT it's exactly this (matching line speed close to the minimum possible) which has helped me to catch several of my best fish!
Besides catching the smart fish there are quite some disadvantages coming in by a significant extra in line speed when fishing:
Increasing loss in control about rod hand and line hand movement as well as the adjustment of both. The more speed we use, the harder it gets to control these key elements.
More tangles reach the first guide when shooting line, because the line has less time to untangle.
Increasing a curved line path into the first guide when shooting line. This increases friction a lot.
Many fly casters are starting the next cast with a line sag in the unrolled line of the current cast. Increasing line speed will pronounce the line end to follow this line sag downwards. Especially when fishing heavily weighted flies it's the extra line speed which causes further trouble (fly catching the grass behind for example) here! Yes, I know this is hard to believe. Most fly fishermen believe more speed will help here. But no, only proper technique can!
Increased friction between the fly line and the rings. Then we of course have to fight this with even more speed.
Since the air resistance increases in square to speed, we have to fight an increased air resistance against the rod, the line and the fly when using any extra line speed.
Of course we will be much quicker tired, when adding additional force to create more line speed. Soon we increase a loss in concentration, too. Long fishing days are getting extra tough here.
No need to tell, that we have more risk to spook fish when hitting the surface with too much line speed.
All in all it's much smarter to learn to match line speed close to a minimum. A tiny little extra is ok, especially when fishing in windy situations. But no more than that and better don't get used to it! The next day easily may have no wind asking for just the minimum necessary line speed to support your fishing best!
May the speed be with you!
All my best,
Ok, I have to meet a huge Sea trout, right now! ;)