I could be wrong, but I don't think it's the app. I get pretty similar performance on the website and the app. I imagine that it has to do with the volume of data on the back-end.

I will specifically exclude the ipad app from my above comment. That thing is a BEAST. It's the largest app size-wise that I have ever installed and it doesn't do anything but provide a front end to delta.com (most commands send you through a browser to delta.com).