The action of a fly rod is determined by the selection and shapes of the carbon fibre cloths and the resin binder that are used to construct the rod blanks.  Generally a faster rod requires a higher degree of casting skill for its properties to be fully exploited. In the hands of a proficient caster it is possible to form tighter loops and throw longer lines. A slow or through-action rod is  better for beginners who are developing their casting skills.