It is surprising that when filling the thumbnails with VGScene that the thumbnails are created nearly instantaneously... Much faster than ImageEn. This maybe cause because VGScene does not create components like the vcl does.
I was investigating mac development, but I am not quite sure it is worth the 9% of mac users compared to the 80% Windows. Maybe you can create an option to load multiple image formats for MAC (no GUI) I think it will be a good start.