Cloth simulation is a critical technique for making films, computer games, fashion design systems, and garment-related e-commerce systems. All those applications need faster and more accurate simulations. This paper presents an optimized and time-critical cloth simulation framework. Several improvements are suggested for major parts of cloth simulation: an accurate triangle-based cloth modeling, a fast block-symmetric Conjugate Gradient solver, an implicit integration with low damping, and an fast body-cloth collision detection method. Based on these efforts, we construct an optimized multilevel simulation scheme. With the proposed methods, we achieved about three times more faster simulation speed than the previous techniques while maintaining the same level of quality.