Non list types are represented using python objects, so you can't really change the precision.
Vectors types lists in Animation Nodes (i.e. Vector3DList, Vector2DList, EulerList,...) are composed of float typed numbers, which is the smallest possible type, so you can't have smaller precision than that.
A float list in Animation Nodes is actually composed of doubles, so you can reduce the precision by using a
An integer type in Animation Nodes is a long, so you can reduce the precision by using a
CharList, or an
IntegerList. Though I believe two of them might have the same precision on some systems/compilers.
Finally, I think it is unlikely to get a performance boost out of this, at least in the context of AN. So I think you are better off optimizing your node tree somewhere else.