Version of LR?
OS and Version?
Format of the images?
Does it happen right after import or anytime later, and only on the first selecting and stays afterwards, on subsequent selects?
Do you see the word Loading at the bottom of the image when this is happening? If you do that is LR loading and or making a full screen preview of the image as apposed to the displaying the included thumbnail in the filmstrip or in Grid view. It also might be applying a preset you have assigned.