Redundancy techniques have been extensively utilized to enhance the reliability of engineering systems. There are three different types of standby techniques, cold, hot, and warm. Warm standby is adopted for less energy consumption and shorter leading time compared with hot standby and cold standby, respectively. Besides redundancy, performance sharing is another strategy to enhance system reliability, where the subsystems with sufficient performance can share the surplus performance with other subsystems with deficient performance. This article considers a demand-based warm standby system with a common bus performance-sharing mechanism, where the subsystems can share performance through the common bus and each subsystem can be configured with warm standby components in order to meet its demand. To be more general, the imperfect switching for the activations of warm standby components is also considered. Moreover, the multi-valued decision-diagram technique is developed to analyze the reliability for the proposed model. The proposed technique can handle systems whose time-to-failure distributions can follow arbitrary distributions in addition to the common utilized exponential distributions. Numerical studies are provided to validate the proposed model and technique.