Realistic randomly deviation of production speed/time

Now all buildings are working with constant specified speed and all balanced chains are running like clockwork.
But in the real world any production works with some fluctuations of speed/time (80-100% of effectiveness) because of human factor, maintaince and supply issues. And it has an interesting effect for long chains, because delays can only grow on each link and not reduce.
The sugestion is to add the random delay (speed reduction) for each production cycle in some range, for example 80-100% of specified speed.