This is a moving target. There are different limitations for iPads with different processing power – for example, the iPad 2, the first retina iPad, is notoriously slow. The best advice I can give is to keep your documents as small as possible. Use SVG images whenever possible, and avoid full page image animations. Test on as many devices as you can get your hands on!
I’ve compiled everything I know about iBook widgets here: http://forums.tumult.com/c/exporting-previewing/ibooks-widget