A fly-rod has a natural frequency (see illustration) which is the number of vibration cycles per unit of time. It is usually expressed in Hertz in the technical domain: 1 Hz (Hertz) = 1 cycle per second.
Some people prefer to use “cycles per minute” (abbreviated to cpm), and 1 cpm = 60 Hz.
When testing the natural frequency of fly-rods, one can find figures varying from below 2.5 Hz = 150 cpm to more than 3 Hz= 180 cpm. To express this in casting terms a natural frequency of 180 cpm gives a cycle time of 0.3 seconds from when the fly-rod is initially loaded to when it is straight (after going through full flex and counterflex. So one can speak of “slow” rods (less cpm) by comparison to “fast” rods (more cpm), depending on their number of cpm. This natural frequency is linked to the ratio of the stiffness of the rod divided by its mass: the higher the stiffness and the lighter the rod are, the faster it is. Every mass put on the rod (wraps, guides, varnish), especially on the tip, contributes to slowing the rod. Some blanks have been measured at 4 Hz (240 cpm) before hardware mounting. On top of that, when you cast a line, the mass of the line also contributes to slowing the rod. To give an order of magnitude, rods loaded with 30 feet of line are considered to be “fast” if their (loaded) frequency is above 90 cpm; and “slow” if this frequency is below 75 cpm. Most rods have a loaded frequency with 30 feet of their prescribed line in the range 78 / 87 cpm. You may not realise it but any caster can perceive a change of 2 to 3 cpm and some trained casters are even able to detect one cpm. Most of the time, casters have a preferred range for cpm and its variation with line length, which is linked to their casting style.