Cubase 5 slow audio mixdown

I used to be able to export midi tracks to audio very quickly. Now, without, as far as I can tell, changing anything, the export time is much longer, in the order of minutes rather than seconds. Also, the progress slider doesn’t appear, so I have no idea of the progress of the export. This happens in existing projects, no matter how many virtual instruments are loaded (in any case, I tend to bounce track by track so that I can make any adjustments before bouncing down the whole project). But when I open a new project, I get the proper, very fast mixdown speed I used to have, and the progress slider works.Processer load and RAM do not seem to be an issue.
Any suggestions?