If you are using LR6 there is a new Auto setting in the prefs.
Many users find standard previews acceptable for their work. Others prefer to generate 1:1 previews on import. This is often important for studio work or other focus critical photography where it’s necessary to check sharpness whilst deciding on picks and rejects.
If you are using LR6 / CC 2015 I suggest to set it to Auto in the preferences. This settings looks at your display resolution and selects an appropriate value.