Generally slow loading and choppy performance is caused by heavy images. Many users make the mistake to choose 300ppi when preparing images and exporting an epub when they picked the iPad or other tablet templates in InDesign.
This is en entirely wrong workflow, since InDesign expects an effective 72ppi. In simple terms, ensure you are working at the native resolution of the main deployment targets' screen resolution, or close to it.
So for a typical iPad a 2560x1536 target resolution is what should be kept in mind. A full screen image should adhere to that resolution. Then when exporting the epub, select 72 ppi. NOT 300 or 150, or whichever, because you run the risk of exporting graphic elements which are then converted to bitmap elements at more than 4 times bigger than the required base resolution. Which will not only blow up (more than quadruple) the size of your epub in worst case scenarios, but also tax the devices resource FAR more, trying to shift quadruple the pixels (because all graphics are stored in the video ram as uncompressed images!).
First thing to check is the resolution of your assets in the exported epub.
And be aware that InDesign is notoriously bad at optimizing bitmap assets. Best practice is to do a last manual optimization run after the final epub version is exported from InDesign. It can mean a 50% reduction in book size, which means more content can be delivered as well.
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