How does the efficiency of multi-tasking differ by task type? My understanding is that multitasking impairs efficiency for cognitively challenging tasks.
Does this apply to trivial tasks as well?
First, it is important to disambiguate between concurrent multitasking and sequential multitasking (Salvucci et al, 2009). In a recent publication I present a short overview on related work of both (Jeuris and Bardram, 2016):
During concurrent multitasking cognitive resources have to be divided across several competing parallel tasks, such as driving while talking on the phone. Many ‘dual-task’ studies investigate dual-task interference and reduction of performance while performing two simultaneous tasks. ...
In contrast to concurrent multitasking, sequential multitasking denotes the interleaving of several primary tasks which are executed one at a time. ... Studies investigating sequential multitasking are among other things interested in measuring the effects of task interleaving on productivity and accuracy. ...
Results show an inverted U-relationship between multitasking and productivity; there is thus an optimal amount of task switching which leads to the highest productivity. However, increased levels of multitasking lead to a significant loss in accuracy, indicating a trade-off between productivity and accuracy (Adler & Benbunan-Fich, 2012).
In regards to the effect of task difficulty on sequential multitasking, "subjective task difficulty has been found to be a determining factor: easy tasks benefit from multitasking by increasing stimulation, whereas hard tasks decrease performance as the result of an overload in mental workload (Adler & Benbunan-Fich, 2015)".
So to answer your question, at least one study has found that for the tasks they used more challenging tasks do 'suffer' more from multitasking. However, there is still an optimum amount of multitasking.
Salvucci, D. D., Taatgen, N. A., & Borst, J. P. (2009, April). Toward a unified theory of the multitasking continuum: From concurrent performance to task switching, interruption, and resumption. In Proceedings of the SIGCHI conference on human factors in computing systems (pp. 1819-1828). ACM.
Jeuris, S., & Bardram, J. E. (2016). Dedicated workspaces: Faster resumption times and reduced cognitive load in sequential multitasking. Computers in Human Behavior, 62, 404-414.
Adler, R. F., & Benbunan-Fich, R. (2015). The effects of task difficulty and multitasking on performance. Interacting with Computers, 27(4), 430-439.
Adler, R. F., & Benbunan-Fich, R. (2012). Juggling on a high wire: Multitasking effects on performance. International Journal of Human-Computer Studies, 70(2), 156-168.
The effects of multitasking are apparent even for the most trivial tasks. For example try walking and eating a sandwich at the same time, you will walk slower and more irregular. The multitasking effects though, are much less when the competing tasks are simple or familiar.
Exctract from this article :
In experiments published in 2001, Joshua Rubinstein, PhD, Jeffrey Evans, PhD, and David Meyer, PhD, conducted four experiments in which young adults switched between different tasks, such as solving math problems or classifying geometric objects. For all tasks, the participants lost time when they had to switch from one task to another. As tasks got more complex, participants lost more time. As a result, people took significantly longer to switch between more complex tasks. Time costs were also greater when the participants switched to tasks that were relatively unfamiliar. They got up to speed faster when they switched to tasks they knew better.