Fab operations management strives to decrease cycle-time (CT) for driving low inventory, improved quality, short time-to-market and lower cost. This work studies factors contributing to production variability, and evaluates the variability's influence on CT. It relies on queueing networks, CT and variability approximations, operational curve modeling, and common practice. It demonstrates that increasing variability drives longer CT at a growing pace, and has a larger effect on CT than utilization. Growing machine inventory weakens the impact of utilization on CT and almost eliminates it at high inventory, while the impact of variability on CT remains significant. Decline of machine availability prolongs CT at a growing pace, and is affected by variability more than utilization. Overall the primary factor of production variability is attributed to machine availability, and specifically to repair time. Reducing variability for achieving decreased CT is less costly and more effective than reducing machine utilization or increasing capacity.