What is a high-speed circuit

It is generally considered that if the frequency of the digital logic circuit reaches or exceeds 45MHZ~50MHZ, and the circuit working above this frequency has accounted for a certain amount of the entire electronic system (for example, 1/3), it is called a high-speed circuit.
In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself, which is the unexpected result of the signal transmission caused by the rapidly changing rising and falling edges of the signal (or signal transition). Therefore, it is generally agreed that if the line propagation delay is greater than 1/2 the rise time of the driving end of the digital signal, such a signal is considered to be a high-speed signal and produces transmission line effects.

The delivery of a signal occurs at the instant the signal state changes, such as rise or fall time. The signal passes a fixed period of time from the driver to the receiver. If the transit time is less than 1/2 of the rise or fall time, the reflected signal from the receiver will reach the driver before the signal changes state. Conversely, the reflected signal will arrive at the driver after the signal changes state. If the reflected signal is strong, the superimposed waveform has the potential to change the logic state.