When exporting content as part of a slideshow, Lightroom factors in any filters or image adjustments made to the Library asset. So the exported image is a web-optimized duplicate of the image in its current state.
For Lightroom 2 or newer users, you do have the additional option of applying sharpening to your images when downscaled and exported. Look for the "Sharpening" slider underneath the image size sliders at the bottom of the plugin's settings.