Hardware Q's:
-you say your proc is 1.6 ghz, but is it a P2 or P3, or P4, or AMD?
-What is your mobo's buss speed?

Anyhoo, displacement would be my first guess as to why it may be taking to long.
Also, do you have a screenshot you could show us consering your displaced object(s).

More than one displaced object obviously will add to the calculation time too.

..sry man, just shootin in the dark here