It’s simple.
Say your GPU has 4 cores and each core is capable of processing 64 images in a sec. And all of the 4 cores work in parallel.
So your GPU will take the same amount of time to process 64 images and 256 images.
So to process 256 images, it will 1 sec if you give batch size as 256 but will take 4seconds if you give batch size as 4 and will take 8 seconds if you give batch size as 32.
Try Giving batch size as 1 and it will take even more time