Also, don't go overboard on modifying the terrain. Like, flattening a huge area to build a structure.

Every time you modify an area it will save as a 'Instance' in the world. When the instances start to pile up from too much terraforming, it will make even the best computers crawl.

You can the hit "F2 key" to few how many instances you're loading in your area. Once it starts going above 8,000 instances, you will start to feel it.