Prototype optimization - jpg or png better?

Hi,
I have a prototype that needs to be tested on mobile devices and is awfully slow and occasionally crushing there (fine when previewing on desktop). Done most of the regularly recommended things like removing invisible layers, fills, necessary frames and component variants, downsizing images to exact size I really need, shadows out, smart animate only where really need etc. Still not really good, so I wonder now: resolution of my images is small, but they are png. Does anyone know, would that affect protype performance? Would jpgs be any better?