We need a little history to make sense of fly rod actions. The first so-called fly rods were wood, and pretty crude. Fly lines were braided horse hair, not very effective either. Eventually rods evolved to cane, then split cane into intricately designed casting tools. Works of art - then and now.Split bamboo soon became the ultimate fly rod and still today is used by traditionalists and collectors.
During World War II, many new products hit the market. Fiberglass was one of them. So was PVC. Dacron and Nylon came out of the war effort too. Lots of innovations that ended up in the fly fishing world.
Fiberglass fabric was wrapped on a wood rod. Epoxy finished and the fiberglass rod was on the market. When the inventors figured out how to do away with the wood center the first hollow rods were born.
Considering the choices available at the time, either split bamboo or steel, the fiberglass rods were designed to copy bamboo. Fake bamboo rods. With a similar action ... as close as possible. Cane fly rods have a soft to moderate action. Some fiberglass rods were very soft and often referred to as "noodley" and the longer the rod, the more noodley it became.
Follow through a couple more decades into the space age and we find the fly rod industry taking advantage of new technology again. This time it is graphite and boron. Almost any material can be formed into threads of some sort. Once it's thread, it can be woven into fabric ... just like fiberglass which appeared in drapes, patching material for cars and boats - and flyrods. Boron was tried and rejected because it was considerably heavier than graphite without additional strength.
Here comes the catch. Those companies who had been making fiberglass rods had everything set up to make those rods to imitate or at least be similar to bamboo. Now there is a new material. What will happen?
Most companies had their machinery, mandrills (steel forms the fabric is wrapped on to make the blanks,) and their reputations established. If you bought one of their rods you knew by reputation how the rod would feel.
Some companies replaced part of the threads in the fabric with graphite. That made the rods lighter, but only slightly changed the action.
New companies had the option to make the same slow or medium action rods already on the market - or to do something entirely different. That's where faster action rods started.
Fast rods that are lighter in your hand, use more of the tip to propel the line, and (if cast correctly) do the work for you. One of the major advantages in fast rods is the person casting it does less work. So you aren't as tired as quickly.
Fast rods are primarily designed with the butt and middle sections built to play the fish, the top one third to cast the line. That does make the tip section (which is tiny compared to rods just a few years ago) vulnerable to breakage if the rod is used incorrectly when playing or landing a fish.
Bottom line? What do you want to fish for? Under what conditions? What can you afford? If you are casting well enough to get the fly where you want it most of the time, you also probably have developed a style - a method that is comfortable for you.
From one who has been fishing for some years, a word of encouragement. There is no such thing as a bad graphite fly rod. All of us would have thought we had gone to heaven if we had been offered any of the rods on the market today 25 years ago. The fly rod industry has made giant leaps improving our sport.
Match the action of the rod, and the price, to what works for you. Final word of wisdom: Don't ever buy a rod you have not cast.