Settings - Fade-in effect
In order to work properly, ShortPixel Adaptive Images (SPAI) lazy-loads the images. This means that they won't be loaded until they are needed, which is when they will be visible on the user's screen. In order to make the transition from invisible to visible smooth enough, SPAI fades in the images by default, otherwise, the images would suddenly show up and from a user experience point of view, that may not look good.
If for some reason, you experience problems with images with special effects, or if you simply don't want the fade-in effect, try disabling this option.