So it seems to have been the dress. I noticed a number at the bottom: Spring Count. For the slow dress it was more than 7 million. For the second dress I tried, it was 650,000 (less than one tenth of the previous dress). However, the static simulation on the second seems to have completed a few cycles and then stopped - certainly not fully draped - yet VWD doesn't indicate that it has stopped and the Task manager is still showing 75% CPU.

There does not appear to be any indication of progress or even that it is working so I have to keep trying to see some movement in the dress - which is not happening.