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Problems with scrolling in 2d Game (D3D 8)

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Ok here''s the problem The engine works fine, and scrolls around fine and crap... buuuut, whenever I scroll, the framerate gets cut in half. It''s an RTS game so scrolling works by moving hte mouse to the side of the screen, like Starcraft or Command & Conquer. So anyways, I thought it might be the timing code, but it doesnt seem too bad (I posted it below). I ran it through the MSVC 6 profiler and came up with something weird (I spent the whole time in the game scrolling, so the framerate was quite low -- 46 compared to ~100 normally - so I could get an idea of what was causing the slowdown):
        Func          Func+Child           Hit
        Time   %         Time      %      Count  Function
    5907.681  34.6     5907.681  34.6     1039 CDirect3D::Flip(void) (game.obj)
    2736.334  16.0     2736.334  16.0   710552 CDirect3D::BlitCustom(struct TLVERTEX *) (gamemap.obj)
CDirect3D is my Direct3D wrapper class. Flip() just calls Present(). BlitCustom is what I use to call DrawPrimitive for every tile on the map. It takes up about as much time as I expected it to. But the Flip() function takes up more! I know that Present() has to wait for the device to become free or whatever, but this seems pretty ridiculous. Now I don''t do dirty rectangle updating or anything; the map is redrawn every frame, so I have no idea why it slows down so much when I''m scrolling. Someone suggested it might have something to do with the hardware. I''m using a GeForce2 MX PCI (shut up). Here''s the main game loop code:
	//Calculate time delta

	QueryPerformanceCounter (&liCurrentTime);
	liTimeDelta.QuadPart = liCurrentTime.QuadPart - liLastTime.QuadPart;
	fTimeDelta = (double)liTimeDelta.QuadPart / (double)liFrequency.QuadPart * 1000;
	//Set last time and add built-up time to execution buffer

	liLastTime = liCurrentTime;
	if (fTimeDelta < 0) fTimeDelta = 0;		//Timer overflow check

	fExecutionTime += fTimeDelta;

	//Update mouse position

	POINT cursorPosition;
	GetCursorPos (&cursorPosition);
	iMouseX = cursorPosition.x;
	iMouseY = cursorPosition.y;

	//Run through logic loop

	while (fExecutionTime > UPDATE_TIME)
		//Get input

		directInput.ReadKeyboard ((CDirectInput::CInputHandler *) &gameInput);

                //Update units and stuff here

		//Handle timing

		fExecutionTime -= UPDATE_TIME;

	//Handle scrolling

	if (iMouseX > direct3D.screenWidth - 10)
		gameMap.iTopLeftX += fTimeDelta * SCROLL_SPEED;
	if (gameMap.iTopLeftX > gameMap.iMapWidth * 32 - direct3D.screenWidth)
		gameMap.iTopLeftX = gameMap.iMapWidth * 32 - direct3D.screenWidth;
	if (iMouseX < 10)
		gameMap.iTopLeftX -= fTimeDelta * SCROLL_SPEED;
	if (gameMap.iTopLeftX < 0)
		gameMap.iTopLeftX = 0;
	if (iMouseY > direct3D.screenHeight - 10)
		gameMap.iTopLeftY += fTimeDelta * SCROLL_SPEED;
	if (gameMap.iTopLeftY > gameMap.iMapHeight * 32 - direct3D.screenHeight)
		gameMap.iTopLeftY = gameMap.iMapHeight * 32 - direct3D.screenHeight;
	if (iMouseY < 10)
		gameMap.iTopLeftY -= fTimeDelta * SCROLL_SPEED;
	if (gameMap.iTopLeftY < 0)
		gameMap.iTopLeftY = 0;

	//Render frame

	RenderFrame ();
liCurrentTime, liLastTime, and liTimeDelta are all LARGE_INTEGERS iCurrentFrame is an unsigned int fExecutionTime and fTimeDelta are both doubles. Any help is greatly appreciated.

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Here''s the profiler output when I don''t scroll:

Func Func+Child Hit
Time % Time % Count Function
2617.455 19.8 2617.455 19.8 589304 CDirect3D::BlitCustom(struct TLVERTEX *) (gamemap.obj)
2251.122 17.0 2251.122 17.0 70 CLogFile::operator<<(char *) (logfile.obj)
1976.691 14.9 1976.691 14.9 829 CDirect3D::Flip(void) (game.obj)

And here''s the RenderFrame() function:

//Make sure display device is working

if (direct3D.CheckDevice ())
//Start drawing

direct3D.ClearBuffer ();
direct3D.BeginDrawing ();

//Draw the map and minimap

gameMap.DrawMap ();
gameMap.DrawMinimap ();

//Draw menu if it is open

if (bMenuOpen)
//Draw the menu

DrawInGameMenu ();

//Draw FPS

char sFPS [50];
sprintf (sFPS, "%i", iFPS);
direct3D.BlitTextFast (sFPS, TEXT_VERYSMALL, 0, 0);

//Draw cursor

direct3D.DrawCursor (iMouseX, iMouseY, iMouseState);

//Finish Drawing

direct3D.EndDrawing ();
direct3D.Flip ();

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Ok.. weird stuff.

I commented the ClearBuffer() in RenderFrame() because I don''t need it (in fact I thought I had commented it out earlier, DrawMap() draws over everything on screen) and the whole scrolling slowdown business stopped.

Any ideas as to what happened?

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