2D application development - performance related query

Dear All,
                I am an application developer intending to use OpenGL.
I understand OpenGL works with 3D geometry Only and all 2D geometry are
treated as 3D with z=0; Is this understanding correct?
Is there any other API for 2D application development which is
performance wise better over OpenGL?
This is in the context of development of a viewer application for
display of semi conductor layout geometry containing millions of
polygons and all present on a 2D plane.

Thanks in advance.

regards
naren