Shopify Latest Photo Issues (size guide and grainy photos issues)

Hi there, I’m new to Shopify and as I’ve recently talked to one of the support staff they told me that Shopify will now optimize and compress your photos further, meaning as long as your photo is below 20mbs or 20 megapixels, no further complicated resizing are required like the old days. I’m unsure if this is true and wondering for example between a 15mb and 5mb file size of the same photos, once optimized further by Shopify, will there be much difference in terms of performance and load speed? (I’m imagining once compressed down to 500kb or 100kb there will still be a difference).
Additionally I’ve notice some of our products photos which have been added grain effects that seems to impact the Shopify final photo in a bad way, photo’s quality got degraded significantly when zooming in. Does anyone has previous experience in terms of the grain issue vs photo quality

Regards

Thanks, in terms of my other bits of the question do you personally have some experience and could answer them? The difference between two file sizes of a same photo should I care about them..as well as the grain in photos?

Thanks heaps for your prompt response.