I found the chrome pagespeed tool and I tested it out on my isoptope index page on your theme - my page is loading slow because of my images. The images that I uploaded is 640 x 640 BUT I used the Regenerate Thumbnails plugin like you said to resize images so it should be using the smaller images, no?
Here is one of the suggested image scaling solution:
The following images are resized in HTML or CSS. Serving scaled images could save 1.3MiB (87% reduction).
http://dtjin.htpwebdesign.ca/.../honey_mustard_bbq_chicken_thigh-640x6... is resized in HTML or CSS from 640x640 to 208x208. Serving a scaled image could save 130.2KiB (89% reduction).