media gallery loading slowly

Hi all, I've got the latest version of Fotografie Pro installed. WP is up to date. Since a few weeks the media gallery is loading thumbnails very slowly: about 20 seconds to load 40 images, and another 20 seconds for the next 40 etc.... Really annoying. It worked normally before. No idea what the issue is. I tried deactivating a plugin for creating folders in the media library. I installed a plugin to regenerate the thumbnails.... I'm not even sure it's caused by the Theme.... Any ideas? Would be great, thanks!