WooThumbs: Product gallery images are loading slow

I installed "WooThumbs - Awesome Product Imagery" plugin on my WooCommerce site to display the product gallery nicer. I found that the thumbnails generated by WooThumbs are loading slow or sometime I need to refresh the page to have them loaded.

I first thought it was the thumbnails/product image gallery size are too large. But it is about 150kb per image which I think is good ?

This is the link to a single product of my site. Can anyone give me some ideas to improve this ?

Below is the picture which shows it is in loading state:

Thanks a lot, CK

via Chebli Mohamed