I've noticed Lightroom slowing down significantly with large catalogs, e.g. having about 550,000 photos. My guess is it's related to the MySQL database that it works with which is not powerful enough. I'm a wildlife photographer and we tend to take LOTS of photos. It would make sense for me not to delete any photos, but rather flag the bad ones as rejected and keep them in my catalog. However, now that Lightroom struggles with large databases I have to re-think my workflow. Is there any plans to address the issue of slow databases?
Optimize your catalog: File->Optimize Catalog
It would also help if you state exactly what tasks in Lightroom are slow