Slower render buckets update on larger images

Dont know if this is a bug but, i have an interior scene and i’ve set the resolution to 5760x3840 (a large render). And press render. it renders very slow. Then i set a render region in the middle of the image, maybe 1/4 of the original size and press render. it renders considerably faster. why? I render on cpu, and the speed at which the tiles update should be the same if i use render region or not? Could any of you try this on a scene.