What platform are you on? SumatraPDF handles most ebook formats and is nice and zippy.
Ubuntu/Linux Mint. I like to customize the typography, the colors, the text width. SumatraPDF doesn't seem to be available on Ubuntu though.
Apparently people have had success [1] with Wine, but that's presumably not ideal. It does have some customizability but probably not as much as you're looking for.