crazy speed difference between GL_REPEAT and GL_CLAMP_TO_EDGE (texture repeating)

Hey guys, I've recently hit a huge bottleneck in my program where I didn't think there would be one. I narrowed it down to a simple scenario: * Create an OpenGL texture with the following pseudo-code:

    (glTexEnvi GL_TEXTURE_ENV GL_TEXTURE_ENV_MODE GL_DECAL)
    (glTexImage2D GL_TEXTURE_2D
                  0
                  GL_RGBA
                  width
                  height
                  0
                  GL_RGBA
                  GL_UNSIGNED_BYTE
                  data)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_MIN_FILTER
                     GL_LINEAR)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_MAG_FILTER
                     GL_LINEAR)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_WRAP_S
                     GL_REPEAT)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_WRAP_T
                     GL_REPEAT)

* Create a loop that simply renders a quad that fills the window with the above texture applied to it (so you just see the image rendered in the window). That's basically it. Surprisingly, for a 465x638 window, each loop iteration takes ~.55s! However, here's the kicker, if I change the texture wrap parameters to GL_CLAMP_TO_EDGE instead of GL_REPEAT, I get the expected faster speeds of ~.002s per iteration:

    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_WRAP_S
                     GL_CLAMP_TO_EDGE)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_WRAP_T
                     GL_CLAMP_TO_EDGE)

Is there a simple explanation for this, or is it something in my opengl initialization or drivers? I'm on OS X 10.5 and here's some of my driver information: OpenGL vendor string: ATI Technologies Inc. OpenGL renderer string: ATI Radeon X1600 OpenGL Engine OpenGL version string: 2.0 ATI-1.5.36