Execution time: GetFrontBufferData, GetBackBuffer, GetRenderTargetData

Question

I'm trying several methods to capture screen with DirectX and I've come up with 3 methods so far:

• GetFrontBufferData() - Average execution times:
  • GetFrontBufferData(): 0.83598 s
  • D3DXSaveSurfaceToFile(): 0.0036 s
  • Total: 0.83958 s
• GetBackBuffer() - Average execution times:
  • GetBackBuffer(): 0 s <-- INTERESTING - WHY?
  • D3DXSaveSurfaceToFile(): 0.2918 s
  • Total: 0.2918 s
• GetRenderTargetData() - Average execution times:
  • GetRenderTargetData(): 0.00928 s
  • D3DXSaveSurfaceToFile(): 0.00354 s
  • Total: 0.01282 s

Average times have been computed by taking 50 screenshots and by measuring time with clock().

All the methods above work, however, as you can see, execution times vary a lot from one method to another for the same results (apparently).

So I have a few questions:

• Why the execution time differs so much from one method to another?
• Why is GetBackBuffer() very fast but then D3DXSaveSurfaceToFile() is very slow?

I have an hypothesis about my first question, correct me if I'm wrong:

• GetFrontBufferData() is very slow because the front buffer is in the VRAM and it's locked/protected because that's what's currently displayed to the screen.
• GetBackBuffer() (and D3DXSaveSurfaceToFile()) is faster because the back buffer is in the VRAM but it's not currently displayed to the screen, so it's easier to retrieve.
• Finally, GetRenderTargetData() is fast because the render target is in the system ram (not in the VRAM) so we can retrieve it very fast.

Thank you.

EDIT: I'm still looking for an answer.