[Freesurfer] Recon-all taking 20 hours to run.

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I ran 38 subjects through Freesurfer 6.0 on a supercomputer here at Washu. The super computer is about 3 years old. Of those 38 subjects, 7 exceeded 20 hour time limit on the batch script. The others took about 12 hours to finish. All subjects had a 1mm isotropic MPRAGE and FLAIR. Below is the command I used for one of them. Is there a reason for some subjects recon-all would take more then 20 hours?

recon-all -s 100000016_20170414 -i /scratch/brierm/Images/100000016_20170414_T1.nii -FLAIR /scratch/brierm/Images/100000016_20170414_FLAIR.nii -FLAIRpial -all -mprage -hippocampal-subfields-T1 -qcache