File sizes around 600k for thumbnails are way out of line, and whether they have anything to do with the problem or not, need to be optimized. You can make small, medium and large versions of them for display, or if you want this automated, install Image Handler 2.
Thumbnails should not be larger than 5-20k depending on the situation.

I see that you have the different files for small/medium/large, but you have not optimized any of them for the actual display size. An image displaying at 490x356px should not need 768k, but more like 50-100k for high quality.