The images displayed in this portfolio are almost all over 1 mb in size, and yet they load instantly.
Loading large content fast heavily depends on the server speed. The theme doesn’t have much to do with this issue.
Even based on user experience, the site loads pretty fast, but Google still gives out the Properly size images warning:
Also, can you think of a reason that this page would not collapse the images on mobile?
Can you switch to the mobile preview mode, and select the containers nested in the Grid block, what’s the width set for them?
Let me know!