If you're looking for wallpaper changer for unity and gnome shell. I suggest checking Wallch. It can be integrated with the dash. You get to choose the folders, test wallpapers and exclude any specific wallpaper from the display.
There is also Live earth wallpaper that shows the day and night cycle on your desktop. I didn't found them resource heavy but make sure you have some enough RAM while using this program just in case if you set wallpaper changer on short interval.
Personally I found that 30 minutes to 1hour time is good for changing wallpaper. Feel free to let me know if there are some good wallpaper changer per workspace. That would be cool program to install.