I import with an module my images.
I have one image per product in the size 1100x1100 pixel.
This is the base, thumb and small
When now I open the category view, he display the images (size 300x300pixel) but load the base image with 1100x1100 pixel.
Of course, the loading time is very long, because he load a big image and display it just smaller.
Can I produce for all products the thumb and base image automatic?
You can change image size inapp/design/frontend/[Theme_module]/[thme_name]/etc/view.xml
After that, clear and flush cache.
Hope it helps.