There is a limit to how much cognitive load one person can take. Once the cognitive load limit (CLL), or the sum total effect of environmental inputs to one's cognition, is reached, there are little to no cognitive tasks that can be achieved well. More simply, the marginal return after the CLL is reached quickly diminishes.
The CLL is constrained primarily by an individual's physical energy. Each individual will have their unique CLL and when they hit that point. An individual's CLL will also change over time as age sets in, or if there are other cognitive activities at home that compete for cognitive attention.
Knowing one's own physical energy and CLL is key to management. This is why there are maker's, manager's, and traveler's schedules. These schedules optimize for activity based around CLL and how to remain productive during available times.
Another key to managing CLL is task discipline. Once a cognitive function is performed, no more cognitive energy should be devoted to revisiting that task. This saves cognitive energy for future tasks.
The last key to managing CLL is allowing ample recovery periods. Recovery periods are unique to the individual and also changes over time. Monitoring and allowing recovery periods ensure that cognitive energy is applied relatively equally across a series of tasks, rather than tapering dramatically towards the very end.
Diminishing returns on cognitive tasks will inevitably happen. But the key is to manage this so that the delta between performance on the first versus last task is minimized.