Unity is made to work with the latest and greatest. It is forward looking
and not made for older hardware. That is where Unity 2D comes in. Another
problem is that it is built on the Gnome 3 framework and I am not sure how
that its optimized for older machines.
My general opinion is that older hardware should stick with older distros
and older versions. Those distros had equipment of its era in mind while
later ones pander to the next generation.
There are exceptions. Some people are willing to compile and tinker to get
the most out of what is available to them.