It is specifically mentioned and illustrated with examples that indeed sometimes it's slower to set this up, because of having to combine the results from the separate parallel processes in the package doParallel.
Reference: http://cran.r-project.org/web/packages/doParallel/vignettes/gettingstartedParallel.pdf
Page 3:
With small tasks, the overhead of scheduling the task and returning
the result can be greater than the time to execute the task itself,
resulting in poor performance.
I used the example to find out that in some case, using the package resulted in 50% the time needed to execute the code.
