Been setting up a form with CRED with a repeater image field.
It seems that when an image is added, it's uploaded like you would assume, but when the form is saved, each image is uploaded again. With several large images on a form, that can cause a timeout.
Also, I use a plugin called "Resize images before upload" (http://wordpress.org/extend/plugins/resize-images-before-upload/) and it seems to not work with CRED.