[DiapDealer]

To my knowledge, Sigil simply doesn't have the ability to make an epub "load slow." If it validates with epubcheck, then it has to be a Nook quirk that needs accounting for.

Perhaps it's the cover image itself? Many rendering systems have issues with how jpegs get created/saved (compression, optimized/not optimized, progressive/not progressive, smoothing, exif/xmp data, etc...). The size of the image in the attachment you added itself is quite excessive. It alone is much larger than most full novel-sized epubs are altogether.