P: Impromptu slideshow in develop module - first image very pixelated

Lightroom 6: When running an Impromptu slideshow from the develop module - the first image looks very pixelated - like an upsized thumbnail, very soft, with no fine detail at all. Other images in the slideshow look like 'normal' slideshow images - still unacceptably softer then the image is displayed in the develop module, but much better than then first image. Playing the slideshow from the slideshow module generates 'normal' slideshow images. Also an impromptu slideshow from the library module does not show the problem on the first image. Changing the slideshow quality setting appears to make no visible difference. Also turning on or off use of the graphics processor makes no difference.