Amdahl's law

If part of a job has to be done one step at a time, adding more workers can only speed up the rest. If 5% must run alone, you can never go more than 20 times faster, however many workers you add.

This is the sum behind running many agents at once. Ten agents writing in parallel still queue for one person to read, one merge to main and one deploy. That single-file part sets the limit, and past a point another agent mostly adds cost.