Parallelisation may not be always better than sequential processing

Parallelisation incurs some overhead: information needs to be distributed over the nodes, and the result from each node needs to be collected and aggregated into the resulting object. This overhead is one of the main reasons why in certain cases parallel processing takes longer than sequential processing. Read more at:

http://www.r-bloggers.com/parallel-processing-with-short-jobs-only-increases-the-run-time/