Hello,
It would be good to know what sorts of things you will be doing with this setup? If you are mainly viewing and plotting or using it to do small projects you could probably get by with the 1.2 GHz all of the time. If the project is Xref heavy and large, you will probably speed up your CPU frequently. As others have said, your experience will vary, but you will probably have your CPU running up to turbo during heavy operations. The machine you are using, has an Intel card right? If you are on an Intel card this set up will probably show lag. You could push a lot of the work off from the CPU to GPU if you had a nice spec gaming card or certified card (zooming, panning, etc). I think it is going to feel laggy on bigger files or files with large xrefs with an Iris card and that CPU.
When we used to certify entire laptops, the lowest clock speed that could pass our certification test that I can recall was a 2.3 GHz laptop. It is a very 3d heavy certification though. I can't be sure on this set up because it is not a certified set up.
Dan
AutoCAD Quality Assurance Manager