The landscape in the video is most likely achieved using third party software such as World Machine, World Creator, or Scape (great free alternative). Using these softwares you can generate a whole heap of things, not just a heightmap but slope maps, erosion maps, etc. and that’s what gives you all the fine details you see in that landscape. On top of that, using landscape materials that change based on the height, slope, and so on, will make it look even prettier. For landscape materials, you can look to the marketplace, or look through a lot of different tutorials on these forums. There’s even a few landscape materials that people have made for free here somewhere 
Regarding the lag…t hat is most likely due to your GPU. A 550 is pretty old these days and things like foilage and trees can be very taxing on them.