The Smart Scan I started this evening just lasted 3 minutes.

HDW
Interesting. Something definitely off here then.
A lot would depend on the users system, how many hard drives and the amount of data, etc.
Also if you don't have a network setup or some other things which would otherwise be included in the scan (don't have GrimeFighter, etc.), they would all have an impact. If you continue to use your system, browsing, etc. whilst the scan is running.
But I don't believe all that should take it from minutes to hours - given what your individual scan times were previously.