First, you may very well have been running into memory issues without realizing it. Less memory means more reading and writing from disks which means slower computations.
However, your report that you can’t converge the SCF with the DF approximation is disturbing. Are you absolutely sure you used Psi 1.3.2 for this and added the hydrogens? If you did, please upload the output file so we can investigate. If you didn’t do those things, try again with them.
(Note to self: This may be a problem with SCF reading if the displacements are large, because we need to re-orthogonalize MOs once we figure out orbital reads in DDD.)