Wallpaper Engine is optimized for performance by design and allows you to customize its performance impact. Most wallpapers will not have any noticeable effect on most computers, however the performance depends on how complex the specific wallpapers is.
You can configure Wallpaper Engine to pause or completely disable the wallpapers while playing games (or all fullscreen / borderless-fullscreen applications in general). All performance settings can be found in the "Performance" tab in the Wallpaper Engine settings.