MATLAB parfor is slower than for -- what is wrong?

the code I'm dealing with has loops like the following:

bistar = zeros(numdims,numcases); 
parfor hh=1:nt       
  bistar = bistar +  A(:,:,hh)*data(:,:,hh+1)' ;
end   

for small nt (10).

After timing it, it is actually 100 times slower than using the regular loop!!! I know that parfor can do parallel sums, so I'm not sure why this isn't working.

I run

matlabpool

with the out-of-the-box configurations before running my code.

I'm relatively new to matlab, and just started to use the parallel features, so please don't assume that I'm am not doing something stupid.

Thanks!

PS: I'm running the code on a quad core so I would expect to see some improvements.