It is also worth noting that multitasking is usually a bad idea from a purely process oriented point-of-view too.

A person who multitasks interleaves tasks belonging to different projects. This pushes the end times of all the tasks, except the first one, further into the future.

If some of the tasks are on the critical chain (improved way to calculate critical path) of one or more projects, then the end dates of the projects those tasks belong to, will also be pushed into the future.

You get really big project delays that way.

It is the same thing that happens if you start too many applications at the same thing on your computer – the system gets overloaded, and suddenly everything slows down to a crawl. Whether the system consists of humans working together, is a computer system, or a manufacturing line, does not matter. The system behavior is the same in all cases.