Creating a New Level

Check out this awesome tutorial by World of Level Design. https://www.worldofleveldesign.com/categories/ue4/landscape-heightmap-guide.php

Once you have imported your landscape, we highly recommend loading up your entire level and all sublevels, selecting all the LevelBounds in your Outliner, and DISABLING "Auto Update Bounds". By doing this, your map will have improved memory performance, however, you will have to be extremely careful when placing assets into your map. You must ensure all assets are being placed on their correct SubLevels or Tiles.

TIP

See Step 1 in this tutorial for steps on how to do this: Optimizing Map Tile Boundaries