Why the /fix of nmea_serial_driver is published 1 second late and which time reference does driver use when the parameter time_ref_source is not declared?

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nmea_serial_driver publishes /fix with more or less one second delay in my system, i.e. when a new value is published, information is already more than 1 second old. Wat causes 1 second of lag? Also, what is the usage of the parameter time_ref_source (string, default: the value of ~frame_id) , what does the default part mean? If I do not indicate anything, what time source does nmea_serial_driver use when it publishes a message, GNSS time or the ros time? I have read this answer but it does not explain why there is a delay in between the measurement arrival and when it is published.

thank you