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.
All wallpapers in Wallpaper Engine are free and there are no hidden costs. The reason why we use phrases like "subscribe to a wallpaper" is because all wallpapers are free Steam Workshop subscriptions.
This means that Steam will remember all wallpapers you are interested in and re-download them automatically if you re-install Wallpaper Engine.
No. Steam is only required to download new wallpapers, upload wallpapers and to install updates but all core functionalities of Wallpaper Engine work without Steam running in the background.
Yes, we don't mind if you run it on multiple computers. Keep in mind that Steam will always sync all wallpapers across all computers unless you rely on local backups only.