1. A subject monitoring system for processing physiological information of a subject, comprising: a pulse oximetry sensor configured to generate a physiological signal, wherein the pulse oximetry sensor detects light attenuated by the subject; and a pulse oximetry processor coupled to the pulse oximetry sensor, wherein the pulse oximetry processor is configured to: calculate a correlation sequence corresponding to multiple lag values based on the physiological signal, wherein the correlation sequence comprises a correlation sequence value corresponding to each of the multiple lag values; for at least one lag value of the multiple lag values: determine at least one threshold value corresponding to the at least one lag value of the multiple lag values, wherein the at least one threshold value varies as a predetermined function of lag values, compare the correlation sequence value corresponding to the at least one lag value to a corresponding one of the at least one threshold value, determine whether the correlation sequence value exceeds the corresponding threshold value, and determine whether the correlation sequence value corresponds to a peak; identify a particular lag value of the multiple lag values when the correlation sequence value corresponding to the particular lag value exceeds the corresponding threshold value and corresponds to a peak; determine a physiological rate of the subject based on the identified particular lag value; and display the physiological rate of the subject on a display.