Speed is distance per unit of time. In an animation, the unit of time is one frame, and so the speed of an object is going to be demonstrated by how far it moves between frames. Acceleration is distance per unit of time, per unit of time. Like, a car accelerates at 10 miles per hour per second. For an animation, acceleration or deceleration is defined by how the distance moved between frames changes.
You obviously don't need to do any math when animating, but you have to consider the concept, and how it applies to what you are animating. For a sword swing, then the swing will start slow as you fight inertia, then increase exponentially until a point. Right now, it looks like that point is being reached too soon, and the sword moves too slowly at the crest of it's arc. If you look at a street fighter animation, or something like that, you will see when they kick the leg goes from bent and retracted, to fully extended and taut. This is because if they added any intermediate frames there, the kick would look slow and weak. The animations don't look choppy or anything though, because there is plenty of smooth build-up to that point, and plenty of reaction to the kick afterwards.
I suggest you try and take out one or more intermediate frames, and see if it gives the swing some more snap. Also, adding in some preparatory and subsequent animations, even if they are slight, will help it a lot.
