Texture Streaming Performance

In my recent SDL 1.3 update I made it possible for the old SDL 1.2 API to be accelerated using texture streaming.

On my system on Mac OS X and Linux, this doubled performance!

Mac OS X

• SDL 1.2 testsprite: 514.22 FPS
• SDL 1.3 testsprite texture streaming with OpenGL: 1259.62 FPS
• SDL 1.3 testsprite2 (hardware accelerated):  3865.16 FPS

Linux

• SDL 1.2 testsprite:  495.48 FPS
• SDL 1.3 testsprite texture streaming with OpenGL:  1244.55 FPS
• SDL 1.3 testsprite2 (hardware accelerated):  2556.85 FPS

On my system the Windows performance got worse!

Windows

• SDL 1.2 testsprite using GDI: 1030.71 FPS
• SDL 1.3 testsprite using GDI: 1077.81 FPS
• SDL 1.3 testsprite texture streaming with OpenGL: 623.08 FPS
• SDL 1.3 testsprite texture streaming with Direct3D: 233.97 FPS
• SDL 1.3 testsprite2 (hardware accelerated with OpenGL): 3027.26 FPS
• SDL 1.3 testsprite2 (hardware accelerated with Direct3D): 4259.48 FPS

Clearly the Windows GDI drivers are heavily optimized for 2D performance, but why is the Direct3D streaming performance so poor?

Here's what I'm doing for OpenGL:
  Texture format:  GL_RGBA8, GL_BGRA, GL_UNSIGNED_INT_8_8_8_8_REV
  Texture update:

    glBindTexture()

    glTexSubImage2D()

    glBegin(GL_TRIANGLE_STRIP)

    ...

I'm not doing anything fancy with pixel buffer objects, I'm just making sure that my data is in the optimal format for processing by the OpenGL drivers.

Here's what I'm doing for Direct3D:
    Device Setup:
    pparams.BackBufferCount = 1;
    pparams.SwapEffect = D3DSWAPEFFECT_DISCARD;
    pparams.PresentationInterval = D3DPRESENT_INTERVAL_IMMEDIATE;

    Texture create:
    device->CreateTexture(width, height, 1, D3DUSAGE_DYNAMIC, D3DFMT_A8R8G8B8, D3DPOOL_DEFAULT, &texture, NULL);

    Texture update:
    texture->LockRect(0, &locked, NULL, D3DLOCK_DISCARD);
    texture->UnlockRect(0);
    device->DrawPrimitiveUP(D3DPT_TRIANGLEFAN, 2,
 vertices, sizeof(*vertices));
    ...

For those who are curious, the full code can be found here:
http://www.libsdl.org/tmp/SDL-1.3.zip

So... does anyone know how to improve Direct3D texture streaming performance?