The environment can produce transient faults in digital circuits, especially nowadays with the new technology development. Fault injection has been widely used to evaluate the hardness degree of circuits in which fault tolerance is a requirement, like aerospace or automotive applications. The magnitude of the fault effects evaluation problem is computationally unaffordable, if an exhaustive test must be performed on a circuit. Several solutions have been proposed to improve the performance of the process. Simulation based fault injection offers slower execution speed, but a great flexibility in the fault injection campaign. Emulation based solution are much faster, but not so flexible. In this paper, the fault injection problem is analysed from a performance point of view, and some optimized solutions are proposed.