Hi Jellyfish,
Hmm, it sounds like you’re seeing two issues. For the slowness on the iPad, does switching between using data and WiFi affect how fast the prototype performs? Are all pages in the prototype slow? If only one page is particularly slow, then is that page fairly widget heavy (e.g. containing large images, tables, repeaters, or dynamic panels with many states)?
Regarding the pinned header not being 100% wide in the browser on the iPad, by any chance do you happen to have the prototype sidebar open or set to minimized? If so, does closing the sidebar completely resolve the issue? If not, could you provide more information about the structure of the pinned header (e.g. is it a 100% wide panel inside of another 100% wide panel)?
If you’re able to send the file then that would help us to take a look. If you prefer not to post the file here, then feel free to email it to support@axure.com so that we can look in private. Thanks! 