I'm working on the problem with slow ad delivery and have it at least partially solved.
1. We have a new advertiser on our homepage for the 336x280 spot. Any advertisers we deal with directly get their ads served by our inhouse server, which is extremely fast.
2. I've changed our 3rd-party rotation from BlueLithium/TribalFusion/ValueClick to TribalFusion/BlueLithium/ValueClick. I'd determined that the slow ads were coming exclusively from BlueLithium, so at least on the first several page views for ad-viewing members, ads will come from TribalFusion (relatively fast) rather than BlueLithium (very slow). It should be noted that the rest of the page loads quickly. It's just the ad that has been taking a while to load.
3. I've written to BlueLithium saying that they've got a major problem with ad delivery. In many cases, I can read a message and click the "Next" button before their ad has even loaded, which results in less real viewership of their ads, and, as a result, clickthroughs. Telling an agency that their performance is hurting their clickthroughs is a good way to really get their attention.
4. We are going to investigate a ban on Flash ads from BlueLithium since many of these seem to be served by DoubleClick, and could very well be the source of spyware installs I'm seeing reported.