Ambient F*****G Occlusions!

As you can probably tell, it has not been a good day for me. It all started this morning when Jason showed me and kirill how to easily plug in an ambient occlusion shader to the existing shader of an object, it is a very simple and yet effective way of getting realistic lighting. I had a go, did some tests, (which are below, one is without the plug in and one with- you probably may tell by the subtle lighting in one), then after these tests i decided to have a go at it for the actual scene that i have come up with, and it worked, looked really nice, and immediately was transformed and i was blown away by it. However, rendering was a nightmare and from here on, i had loads of problems with the appauling amount of time that it took, even for a 640 x 480 size render! This was really confusing me, when after when i had unplugged the ambient occlusion shader it was still taking a very long time to render. I have fiddled with a lot of settings and was still having trouble, for a render size of 64x48, it took 40 seconds!!which is ridicules. I had found out later, that some how the mental ray data for the ambient occlusion was still taking an effect and couldnt find a way of stopping. I tried deleting all history, optimizing the scene size, then went into the hypershade and deleted all unused nodes and repeated nodes, then Chris suggested i deleted all unnecessary objects in the scene, via the Hypergraph and again was still taking a while.
After all these head traumas (upgraded from headaches), i decided to head for home and have another crack at the problem. I tried exporting ONLY the scenes objects and importing into a new scene, and low and behold, it renders quicker! The only reason why i didnt try this earlier was the fact that it was taking forever to render a single frame on a different scene altogether, so i just thought it may have been the network or a bug in the system. Just pleased that it is rendering stills much quicker and at 16 seconds- but only at preview quality tho.