What happens if you increase the
topic_list_thumbnail_resolution_level (which lowers the res) to 2?
I’ve now made 2 the default (was 0, serving the highest res thumbnails!!) https://github.com/merefield/discourse-topic-list-previews-theme/commit/c668985a4ebc94e89f22b516c09ce6b651daf4b9
Bear in mind for this to work you have to have
sidekiq running to prepare the lower resolution thumbnails. For the time they are not available it will serve the higher res or original images and you may run into that issue. That situation should only last minutes.