Looks like loops inside parfor/spmd with tons of iterations are not getting all the benefit from JIT (Just-in-Time interpreter/compiler) which allows for-loops to run faster and not be interpreted very single time step.
I have seen this happen before mostly with cases like this - with many, many iterations of a very simple loop.
To force the JIT into action, the appropriate work-around is to put it in a function - as you have done here.
