We can optimized your images as recommended by those tools but, that means it will not be optimized for the theme. It’s like an options you can’t both have.
The recommendation is like, if it’s displayed at 252×142, then it should be resized to 252×142 from 1607×906 reducing 97%. That works perfectly on a site that never changes dimension.
X theme is responsive that works on mobile and different devices. Please note that there are also retina displays that requires large images. So, if you wish to optimize your images according to their recommendation, then you’re sacrificing many good features.
252×142 could be displayed 500px280 on tablet, or etc. So if you will resize your images strictly to 252×142, it will be pixelated on different displays that scaling up.
1607×906 is recommended size for full-width site and on retina display. But 900px to 1200px are okay too, you can upload images with exact size as mentioned.
Or if you wish, upload images with 252×142 replacing 1607×906 as your tool’s recommendation.
We have come with this guide instead that will help improve performance of your site: https://theme.co/apex/forum/t/customizations-performance/210