the different Matlab processes communicate via the file system in the multicore package. All function input and output arguments are saved to/read from the file system. In your example, the multicore master process will save 100 files of about 8 MB (1 million doubles) to the disk which are read by the slaves. The overhead is in this case clearly larger than the benefit of the parallel processing.
For further dicussion, please use the Yahoo group: http://groups.yahoo.com/group/multicore_for_matlab