Why do some herbicides work quickly and some slowly?

Why does herbicide have the difference between quick effect and slow effect, and what is the reason? Do you have any netizens to explain. General herbicides can be divided into conductive type and contact type according to their conductivity in plants. After the conductive herbicide was absorbed by the weed, it was transmitted horizontally and longitudinally through the transport tissue to kill the weed by root. So this kind of herbicide is slow to kill weeds, but it kills weeds thoroughly and kills the roots. After the contact herbicide is absorbed by weeds, the part that comes into contact with the solution dies quickly, preventing the transmission of the drug in the plant, so it works quickly, but it can't be killed by root. The farming network suggests: if it is to control perennial weeds, you can only choose conductive herbicides, not contact herbicides.