Why do setup violations mainly occur in slow paths, while hold violations occur in fast paths?

Setup violations mainly happen in slow paths because the data takes too much time to reach the next flip-flop before the clock edge. If the logic between two registers is too long or too heavy, the data can’t settle in time, so you see setup violations. On the other hand, hold violations happen in fast paths because the data moves too quickly and reaches the next flip-flop before the previous data is safely latched.

From my experience, you can look at it like this: setup time is about being “too late,” and hold time is about being “too early.” In a slow path, signals arrive late, so you miss the setup window. In a fast path, signals arrive too early, which interferes with the old data. That’s why setup is linked to slow paths, and hold is linked to fast paths.

You should think in terms of clock and data balance. For setup, if the data path delay is larger than the clock period, it fails setup. That’s why we say slow paths create setup violations. For hold, if the data path delay is very small compared to the clock delay, the data rushes in too quickly, and the new data overwrites the old one. That’s why hold violations usually show up in fast paths.