Comment by mcv

It's not so much a branch, but a dependency between subsequent iterations of the loop, which means execution of each iteration has to wait for the result from the previous iteration, slowing things down. But if most of the time j == next_j[i][j], that dependency is often unnecessary. So the branch allows the system to predict that it can be parallelized. But that's only really worth it if the branch condition is false most of the time. Whenever it turns out to be true, the CPU will have to backtrack and redo that work.