@agriggio @sguyader Thanks for checking!
Part of the slowness might come from the fact that some UI elements have to be read from the bundle, which is de-compressed on the fly. A way to check this would be to extract the contents of the AppImage to disk, and run it from the extracted folder. Here are the instructions taken from the official documentation:
sudo apt install libarchive-tools # Or any other method to get `bsdtar`
bsdtar xfp /home/me/Downloads/Some.AppImage
Meanwhile I will have a look at the lensfun db.