"Properly size images" or "Serve scaled images" is a code related issue. Means theme loads larger resolution images then it's necessary.
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Automatically adapting image sizes to different resolution devices helps to load images quicker on the website.
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If theme code does not properly resize images, then larger resolution images will load on the website and will be resized by the browser automatically as in example:

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While if images where properly sized, it would load many times quicker as image would be smaller in size and browser would not need to resize the image before loading it.
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To resolve the issue would recommend to contact theme support or hire an expert developer from here: https://experts.shopify.com/