You've manually disabled the AutoCreate for parallel pools - I presume you're manually creating a pool with a separate parpool statement.
Whether or not parfor can go faster than a serial for loop depends on your underlying hardware for this case. It might simply be that the limiting factor is your disk drive, and trying to read from it multiple times simultaneously does not allow any speedup.
You could try manually running multiple copies of MATLAB and calling readtable from each of them simultaneously to see if they slow down when there is contention for disk access. I.e. run something like this in multiple copies of MATLAB:
I suspect that you will see that as you start more copies of MATLAB, the toc time will increase.