9 is much faster than 8 for most things, including being significantly faster with large vector layers. Download the free Viewer and see what you think (Viewer is a read-only version of 9). Use the latest Edge build of Viewer and you have the latest and greatest from 9.
I just opened an 8 .map file with the latest Viewer... it takes a bit to convert to 9 format, but then it flies. This map is most of the bigger roads in France, plus what seems to be all the roads in cities like Paris, about 550,000 line objects. It pans and zooms effectively instantly.
It does that even in a map that uses, say, Pseudo-Mercator and if the Roads layer is in Lat/Lon and has to be reprojected on the fly each time you pan/zoom. 8 can't touch that performance.
See also the videos in the Gallery page. 9 is phenomenally faster than 8 at big rasters. 9 can open 100 GB files full of images instantly and pan and zoom instantly where it would take 8 a day.