Why are 4K wallpapers lagging sometimes?

Well, I have Wallpaper Engine and I download 4K wallpapers, among other things. My laptop doesn't support 4K but why is it lagging? If it can't display 4K then it just won't display it, right? So why are some 4K wallpapers lagging?

Wallpaper engine or what?

Because your laptop doesn't support 4K. Your graphics card will be overloaded because it can't process the images.

Most wallpapers in this engine are individual video clips that have been put together to form an overlay.

If they were rendered in 4K, then the video decoding engine of your graphics chip must also decode them in 4K and then also downscale them to the native resolution of your notebook display.

Why can he then load some 4K images normally and others jerky?

Yes, I said that in the question

It is lagging because your laptop tries to set it to 4K resolution over and over again