It sounds like you've already taken most of the important steps we recommend to reduce load times.
Changing from PNG to SVG may or may not help. SVGs can often reduce performance because their rendering can be more complex then simple pixel pushing, so that is usually dependent on the images themselves.
To offer more specific advice, we'd need to see the project itself. It is likely that the number of scenes, especially if they have a lot of elements, could be the major culprit in load times. You could potentially break up the scenes to separate Hype documents. But without seeing the document that advice could be wrong!