Once you turn on the option to optimize images in the Autoptimize plugin,
you or your visitors will need to access a page before the images on that page are optimized and served from the CDN.
Unless a page is visited, the images on that page won’t be optimized. This is beneficial as your website won’t use more resources than needed. The flip side to this is that the first user visiting a page won’t see any speed improvement at first.