A DigitalPoint forum thread named Anyones Yahoo ads slow to appear? seems to be right. Since I have the ads on this site in rotation, I normally can tell when a YPN ad will show versus a Google ad. How do I know? It depends on how quickly the ad loads. If it loads quick, it normally is the Google AdSense ad. If it lags a bit, then I know it is most likely the YPN ad. Again, YPN is in beta - so I am sure things will ramp up when it comes out of beta. But some people report as much as a five second load time. Enough reason to be cautious and even possibly go back to Google?
Forum discussion at DigitalPoint Forums.