This paper analyzes the impact of Simultaneous Switching Noise (SSN) on the timing behavior of CMOS digital blocks. The concept of Instantaneous Transfer Function is introduced to interpret noisy signals and perform timing measurements on such signals. It is shown that the average swing during switching is the key parameter to predict the noise impact on the delay of a logic path, whereas the peak of noise is not relevant. The influence of structural parameters such as the block topology is evaluated, and the unpredictable aspect of SSN impact on path delay is highlighted.