Environment - NW6.5 SP3, Netdrive 4.1, WIndows XP SP2 client

We are having problems with Netdrive. When users browse using Windows
explorer, folders containing AutoCAD (DWG) and ZIP files takes ages and the
client appears to be downloading them. Since DWG files are usually large
this takes a considerable time. Other folders with Office or PDF files
display quickly. Any ideas?