Illustation of how distributing work on 4 streams in parallel will delay the end date of all of them, even though it seems like you are making progress in all fronts.

Contrast with the benefits of fully focusing on a single stream at a time, and the impact in has on the end date of 3 of the 4 streams, with the 4th one staying the same.

Opportunity cost calculation - Google Drawings

sotware development planning diagrams of streams A, B, C, D requiring 3 blocks each, and comparing doing 1 block at a time of each vs doing all 3 blocks of A, followed by all 3 blocks of B, etc.