Because there are too many bugs in Canvas Draw 6 for me, I stayed with v5. I'm anxiously waiting for an update, which I hoped would be ready by now. I'd rather see incremental updates more often with continuous bug fixes than wait a long time for a major upgrade, much like apps for iOS.
I just checked Sherwood's problem with a 600 dpi image (just 8.5" x 11") in CD 6.0.3 and it took about 15 seconds for the preview to complete. The same in CDV5 was nearly instantaneous so my results are similar to his, but not as bad. Memory and/or processor differences?