I wrote a simple CUDA based image thumbnailing dongle with OpenCV a while back. https://github.com/NealP/cudathumb It's pretty fast but doesn't expose a good interface via console so if someone would like to contribute that I would be not unpleased. Also you need to have OpenCV compiled with CUDA to use it(which is kind of a nightmare). GraphicsMagick with its multithreaded thing(it should just werk if you enable it) is pretty fast if you run it on a decent CPU, it shouldn't take nearly 115 days. If thats still not fast enough for you you could try to make something with the Intel Performance Primitives package(if you're on Intel CPUs), though the cognitive load imposed by writing it might not be worth whatever speed boost it grants.