Modern GPUs have to put the vertices in vram before they can render them.

That means immediate mode emulation is effectively this

    everyframe:
      a) copy all vertices from user code to some cpu buffer
      b) at draw time, copy cpu buffer to vram
      c) ask GPU to draw

steps A and B are very expensive CPU wise as well as waste memory.

vs OpenGL ES 2.0

    init time:
       put vertices in vram

    everyframe:
       ask GPU to draw