Too much useless animation and effects activated in Unity... Unity3D makes the system extremely slow but only low Graphic Card and/or can actually reproduce the lag and slow animations.

I am in the same case as you because I use an Asus 1215N with a Dual Core ATOM processor and an Intel Graphic card since the Optimus doesn't work natively...

But you can still do some configurations to help a little bit with the problem like this :

When using Unity-3D:

Use the proprietary ATI drivers available in Ubuntu.

After installing ccsm (Compiz Config Settings Manager) :
- Disable Sync to VBlank (as mentioned by @duffydack)
- In Composite Plugin > Select your Refresh Rate (available at Screen Settings) and then uncheck "Detect Refresh Rate".
- In OpenGL > "Texture Filter" select "Fast" instead of "Good", and choose "Texture Compression" (this feature will use a little bite more of the process but will help with graphics so it is at your choose (you can have the 2 options unselected though)

And I think it is all you can do (for me even Unity-2D is slow so I think it is the fault of my processor... even if this problem of slowlyness doesn't occur neither in Gnome-Shell or Windows (but the last one is probably due to the fact that the Nvidia Optimus works and on Linux it doesn't....)

Hope this helps.