Multi-tasking lowers productivity.
What influences on attention cause the productivity of a multi-tasking person to be low?
An example of multi-tasking is that a person is working on his assignment while trying to memorize unfamilar Spanish words.
One important factor to consider is whether the separate tasks involve the same kind of mental process. For instance, among these multi-tasking scenarios:
$\#1$ is generally harder than $\#2$, partly because both of its tasks involve linguistic processing, whereas walking does not.
$\#2$ might be a somewhat unfair example at first glance, because walking is also more self-contained and automatic on flat ground. On uneven ground, however, $\#3$ would be much harder than $\#2$, because both walking and reading require visual processing in such circumstances. This point implicates automaticity and difficulty as (probably related) influential factors as well. I'm sure there are others, and I hope others will help fill in other ideas.