How fast transfer switches are used in minimizing the severity of sags?

Generally transfer switches are used to sense any abnormality in the input voltage and transfer the contacts from the input to an auxillary generator, such that the load is powered in an uninterrupted manner. The transfer switch would transfer the contact to the generator only after the generator has reached the required RPM.
Later when the input voltage raises to sufficient levels and remains stable for a period of time, the contacts are transferred back to the input supply through the transfer switch.

Since we are switching the contacts of input, it has to done as quick as possible in order to avoid surges and transients. In fast transfer switches, this period of switching the contacts is found to be in the range of milliseconds.

Ultra-fast transfer switches are also under development which claims to have transfer period of just a few hundred microseconds. Hope I have answered your question...😎