Why are my images pixelated / have too low quality?
Cause
Your problem is caused by the Optimize tab's Convert to WebP and Resize options. These are recreating your images in smaller sizes, and in some cases that is smaller from what you would actually need.
Solution
Turn off all Convert to WebP and Resize options, and then your original images will be used. (Convert to WebP is only available in the Pro version of Smart Slider.)