Matlab Performance 64bit vs. 32 bit

I have a set of code that runs significantly slower on various 64bit versions of Matlab, e.g. the code runs 2x faster in 2013b in 32bit than in 2013b on 64bit.

This issue seems to still be present in 2014a.

I find this especially puzzling. A bit about the code: it is very for-loop heavy with otherwise very simple manipulations. Any suggestions/explanations as to WHY there is this discrepancy in performance would be useful.