I received an email about a slow plot dialog box in AutoCAD 2006 and I emailed a possible solution from AU last week.
The most likely cause is a plot device is unreachable such as it is offline or not there. The delay should be only the first time you launch the Plot dialog and it checks your system devices. It could also be the PC3 files stored on a network and the networks latency causing the delay.
It turns out the person had a bad PC3 file so removing some PC3 files received from a consultant to a device they did not have solved the slow dialog and now it is speedy again. AutoCAD was following the PC3 file and not finding the device which caused a delay.
Similar to a past Autodesk knowledge Base article TS62597