i have a plane im modelling in maya and importing it into unity. It has only one face but many vertices to create a certain shape. My question is will this slow down the performance similar to having lots of faces/polygons?
It doesn’t just have 1 face. It has multiple. It is triangulated (although this is hidden in Maya).
It will be broken into triangles when you bring it into Unity.
There is only one shape in Unity, triangles.
So, yes, it will have exactly the same effect on Unity as if it were broken into triangles by your modelling app.