There's unfortunately not an easy way to do preloading on a per-scene basis, though this is a popular feature request (at least, to have significantly more control over what gets preloaded and when). There are manual/non-hype methods in which you could do preloading, but it is a bit of work and code to properly maintain this method.
The canonical post to help with general performance issues is:
It sounds like you've already gone through the Resource Library and unchecked preloading on individual resources.
A lot is dependent on the document and your resource sizes. While Hype can try to resize/recompress images in a lot of cases, there's many cases where it either can't resize, or perhaps won't do as good of a job as the performance demands. I'd recommend using Chrome's network tool in its web inspector to see what exactly is taking the most time to load and why - if possible resizing or recompressing any assets if possible.
You're welcome to upload a zip of your .hype document for more specific suggestions to improve performance.