I'm afraid it's not possible for Construct to preview your project without preparing the images. Construct 3 uses an in-editor spritesheeting system which is why it must prepare in the editor, where Construct 2 only did it on export. Since C3's release in 2017 I've not heard of it being a serious issue at all, especially since it only has to do it once after opening your project. It's not doing it every preview, right? Just once after opening, like I described?