So I did some digging. When uploaded as an "Asset Library" (images.squarespace.com) img, squarespace does some magic to reduce the file size (to 380kb from 1.5MB in my test case). It looks like squarespace returns the smallest version of the image while maintaining quality. Interestingly, my duplicate test image uploaded as a CSS Custom File downloaded faster (39ms vs 70ms) despite being much larger in file size (squarespace didn't perform size reduction). Interesting that a larger file downloaded faster, likely due to the server-side logic running on images.squarespace. Also, static1 downloads faster even when there are no query string parameters in the images.squarespace URL. Seems like CSS Custom Files should be prefered for images unless an end user will be using the GUI to manage images.