Adaptivity is sadly still unfinished feature from corona 1.4, so if you want to know what really caused 8x bigger render times you need to know which material is fastes to render and which slowest to render.
It's not just materials, but lighting as well. The attached images are an example, the second two images have a plane in between the light and camera that is bouncing light down on the objects:
img1 - light intensity: 1000, bounce plane: hidden, render time: 1:21, passes: 385
img2 - light intensity: 1, bounce plane: hidden, render time: 1:28, passes: 425
img3 - light intensity: 1000, bounce plane: visible, render time: 2:07, passes: 615
img4 - light intensity: 1, bounce plane: visible, render time: 2:47, passes:825
As you can see clearly, brighter and direct lighting results in faster renders, darker and indirect lighting results in slower renders.
Please post here sampling focus render element, it is possible, that adaptivity is spending most time on material which is hardes to render ( lowest samples/s ) thats why you see so big render time difference ...maybe I am wrong, we will see.
Err, that's what I was suggesting in the first place...