The Plot and Page Setup dialogs (see fig. 1) both include a checkbox for transparency (PLOTTRANSPARENCY), similar to the behavior for plot styles. When this option is enabled, AutoCAD rasterizes the entire drawing for plotting, slowing down the plotting process.
To test if your transparencies are causing the plots to be unacceptably slow, unchecking the Transparency box on the plot dialog and plot. If it plots at a normal speed then you’ve found it!
There are other things that can also cause very poor performance when bringing up the plot and other dialogs. To get help on these and other issues call the NSC at 800-808-7645.