As Mike mentioned, we have introduced new Graphics engine in QuarkXPress from version 10. QuarkXPress no longer generates rasterized low resolution RGB previews for both raster and vector graphics. Also, QuarkXPress uses full power of 64-bit hardware/OS and multi-processing capabilities to give you rich previews with as smooth or fast as 9.x or before. it does generate cache initially, based your hardware there can some lag but it should not interfere with layout operations.
if you are experiencing issues, please share your hardware details and sample projects (with images) so that we at Quark can investigate further. You can drop me an email at firstname.lastname@example.org
. if the files are large (as I would imagine), please share via dropbox or similar. Alternately, please let me know I can provide an alternate location to share files.