Dr.J Chakma said: “1) There can be approximately 600 citta vithi per second if all six senses are actively working, followed by 1800 manodvara citta vithi (3 mind door citta vithi follows every sense door citta vithi). Because human brain takes approximately 10 millisecond to process a sense input. ”
That is true in the case of pancadvara citta vithi. The “time lag” of approximately 10 milliseconds takes for the brain to process a signal coming though the five physical senses. However, the time lag could be smaller, since the brain may be able to process signals coming from the five physical senses in parallel. So, the time lag could be smaller by a factor of two. In any case, these are estimates based on current scientific findings. The actual numbers could change.
Regarding #2: While the brain is processing those external signals, the mind could be in a bhavanga state or in an active manodvara state. It is a complex situation.
The only thing we can say with certainty is that the mind — at a given time instant — will be in just one of the following states: processing a pancadvara citta vithi, processing a manodvara citta vithi, or in a bhavanga state.
Jhana (or phala) samapatti is a special case, where a single manodvara citta vithi can run uninterrupted for hours.