my photos are blurred during slideshow

Are you using iCloud Photo Library and your photos optimized - not stored locally? Then they may appear blurred in a slideshow, if you do not ensure, that all have been downloaded before you start the slideshow.

Look at each photo enlarged in Full Screen, before you start the slideshow.

"Pixels" is the unit of measure for "Picture Elements". The digital images are represented as a grid of discrete elements - the pixels. Each pixel is coding the color and luminance at a certain row and column of the photo. OT's screenshot is showing the pixel size of a photo with width of 3024 pixels and a height of 3024 pixels. For a slideshow your photos need to have at least the pixel size of your display, or they will be blurred, because the will need to be enlarged to fill the screen.

What are the pixel dimensions of the images you're using?

Select a thumbnail and use the Command(⌘) + i key combination to bring up the Info pane:

or use the Window ➙ Info menu option:

léonie's post reminded me of on setting you can try. Make sure this checkbox is unchecked:

See if that will sharpen up the images in the slideshow.