I've noticed high CPU usage and disk thrashing during not only QuickScans, but afterwards as well.
The problem occurs when NIS 2012 decides to update and compact it's databases. Once this process starts it will continue until it's finished. It can take up to an hour (since the databases on my machine are around 75 MB in side). During that time period the disk thrashes continueously and the system (Windows XP SP3) is virtually unusable. This isn't a new problem. It's been happening for a number of years now.
I think the "Idle" detection processing should not only take into account user activity, but CPU and disk usage as well. If there is high CPU or disk activity, the daily database processing should not occur.
Also the database processing should stop if the user tries to use the system.