Preview images only load for first files

@rebeccawatson reports: Interesting connectivity issue (not a bug? I don't think?): When accessing a device/folder contents on mobile, with spotty internet... the preview images load only for the first 12-or-so that fit on the screen. (I have a folder with 40+ images... the preview only works for the first full-screen-worth of images and the rest don't preview)
