The polymer/solvent/nonsolvent systems with different L-L demixing rates were prepared by employing a binary solvent mixture consisting of two solvents - one exhibits an instantaneous liquid-liquid (GL) demixing process, while the other exhibits a delayed L-L demixing process. It was found that an increase in the delay time of L-L demixing results in a denser membrane structure, an increase in fiber mechanical strength, a delay desorption of moisture in membrane, and a decrease in gas permeance, for a hollow fiber fabrication system consisting of cellulose acetate (CA) (polymer), N-methyl-pyrrolidone (NMP) (solvent having an instantaneous L-L demixing property), tetrahydrofuran (THF) (solvent having a delayed L-L demixing property) and water (nonsolvent). Hollow fibers prepared under an instantaneous L-L demixing process tends to have more mechanically weak points (flaws) than those prepared under a delayed L-L demixing process. Surprisingly, SEM observation suggests that membranes wet-spun from solutions containing both THF and NMP tend to have a rough outer skin morphology. Inconsistent demixing and the collapse of the outer nascent skin may be the main causes. In addition, the effect of bore fluid chemistry on fiber performance is much more pronounced for systems having a delayed GL demixing mechanism than that having an instantaneous GL demixing.