Please try to open ISOMETRIC drawing in vanilla AutoCAD. Add check plot preview performance. Will there be any difference in timing? This will let to determine if performance is related to AutoPlant or not.
I assume that the ISOMETRIC drawing is located in network environment. Could you please create etransmit package in AutoCAD, then unpack it local on your machine and test plot preview performance. This action will ensure that the all xrefs are loaded from local storage, thus executing network environment as possible cause.
Please let me know results.
Artiom