Hi,
the regen process is single threaded but normally would not get you to 100% processor loading because it should be disk access limited, although the built-in driver is CPU hungry.
There is one possible catch: it may be that you outran the index space in memory. This text is from the manual and may apply to your system:
"The built-in dBaseIII driver runs out of the box. The parameter IndexDBF in dicom.ini should,
however, be initially set to about 10 times the expected number of million images to be loaded in one
session (the default allows loading 100.000 images). This allocates enough data to store the index
buffer. Spare space is allocated when the server is restarted."
Marcel