Images load erratically on mobile screen sizes

Hello,

Not sure if this is an Upfront or Spirit issue, WP Smush Pro issue, or possibly something else, but the featured images for each post (beneath title, above beginning of post content) are loading erratically on tablets and mobile devices. Most take a long time and all (or most) load with awkward dimensions.

This problem is not apparent at all on default screens (tested on laptop and desktop—makes sense for pixel dimensions associated with each display type within Upfront).

Can you please advise a method to address this?

Thank you,

Clyde