...and interestingly the unforked script does the job in about half the execution time.

...because the unforked process isn't causing your disk heads to seek back and forth repeatedly as each forked process grinds away at the same physical resource.

Sometimes forked processes work like a bucket brigade where the person filling the buckets does it very quickly, and hands them off to the workers who have to run from the water source to the fire and back before they request another full bucket. Other times, it works more like the person doing the filling is standing next to the fire, and has to keep running back and forth between the workers and the water source. You might be in the second situation.