A problem about Index Buffer

I''ve a problem when i using Index Buffer in 3D Mesh. Is it the speed of Index Buffer(IB) faster than that of Vertex Buffer(VB)? Can the size of VB be small after using IB. In addition, eg. DWORD VERTEX::FVF = D3DFVF_XYZ | D3DFVF_NORMAL | D3DFVF_TEX1; thus, i can define a normal vector at each vertex such that i can only use relatively less number of faces to express as smooth surface while i use Gouraud/Phong Shading. However, if i utilize IB, the adjacent of faces will share the vertices in VB. In this way, Is there any conflict between the normal vector and IB? Is it the vertices for different normal vectors duplicating serval times? Can these vertices be shared with the adjacent of faces?