What is ‘zone of frustration’ that startup employees go through?

A Dezerv model compares 2 employees: one paid ₹10 lakh in cash, the other ₹6 lakh cash plus ₹4 lakh in ESOPs. For 8 years, cash-heavy employee stays ahead — the stretch the model calls the zone of frustration, when many equity holders quit. After a year-10 liquidation, ESOP holder ends at ₹2.08 crore, the other at ₹1.05 crore.