First, thanks for putting together the Petfinder Listing plugin. What a great resource for rescue organizations.
I recently modified your plugin for a responsive web design project I was working on. One thing I noticed is that when you request a list of more than 10 or 20 animals, the API really slows down load times. I found a simple way to fix this that may be worth adding to your documentation.
The Quick Cache plugin (http://wordpress.org/extend/plugins/quick-cache/) pre-builds all of your WordPress pages once an hour and serves those instead of the dynamically created pages WordPress word normally create when a visitor comes to your site.
When you use the plugin, visitors are served a pre-built version of the Petfinder listings page, and don't need to wait for the API to load and the page to be created.
You can learn more about the modifications I made to your plugin in this short case study I put together: http://gomakethings.com/about/petfinder-api/.