When things start getting slow are the particles quite large and/or taking up a significant portion of the screen? If so, then it's just simply overdraw and is expected behaviour.

Are you recreating the dynamic vertex buffer every frame? That's likely to cause slowness.

Yep, I think we've found our culprit. MSDN (and your DXSDK) have an article on the proper way to do this; see: http://msdn.microsoft.com/en-us/library/windows/desktop/bb147263%28v=vs.85%29.aspx#Using_Dynamic_Vertex_and_Index_Buffers

Use 2 vertex buffers and switch between them each frame.

Make sure you implement it so that you can expand it to 3 vertex buffers in the future, or 4, or N.

In other words, make it an array and use a macro or enum to determine the size of the array.

L. Spiro

Here's one example of a way of doing this that I coded up (but didn't test/etc), fairly rough and ready but it should illustrate the way things are done. PLEASE don't just copy & paste this code; use it as a way of understanding how dynamic vertex buffers should be handled instead, read the link I posted earlier and see how what's done here matches with what's described there.

class CDynamicVertexBuffer
{
public:
    CDynamicVertexBuffer (IDirect3DDevice9 *device, int maxitems, int itemsize) :
        Buffer (NULL),
        MaxItems (maxitems),
        ItemSize (itemsize)
    {
        assert (device != NULL);
        assert (maxitems > 0);
        assert (itemsize > 0);

        // AddRef the device because this needs to hold a reference to it
        this->Device = device;
        device->AddRef ();

        this->OnResetDevice ();
    }

    ~CDynamicVertexBuffer (void)
    {
        this->OnLostDevice ();

        if (this->Device)
        {
            this->Device->Release ();
            this->Device = NULL;
        }
    }

    void OnLostDevice (void)
    {
        if (this->Buffer)
        {
            this->Buffer->Release ();
            this->Buffer = NULL;
        }
    }

    void OnResetDevice (void)
    {
        assert (this->Buffer == NULL);
        assert (this->Device != NULL);

        HRESULT hr = E_FAIL;

        hr = this->Device->CreateVertexBuffer (
            this->ItemSize * this->MaxItems,
            D3DUSAGE_WRITEONLY | D3DUSAGE_DYNAMIC,
            0,    // FVF is only used if we're calling ProcessVertices, which we're not, so we don't set it
            D3DPOOL_DEFAULT,
            &this->Buffer,
            NULL
        );

        assert (SUCCEEDED (hr));

        this->CurrItem = 0;
    }

    void AddItems (const void *itemdata, int numitems)
    {
        assert (itemdata != NULL);
        assert (numitems > 0);
        assert (this->Buffer != NULL);

        // by default we're just appending to the buffer
        DWORD locktype = D3DLOCK_NOOVERWRITE | D3DLOCK_NOSYSLOCK;
        void *lockdata = NULL;
        HRESULT hr = E_FAIL;

        // check will these items overflow the buffer; if so flush it and rewind
        if (this->CurrItem + numitems >= this->MaxItems)
        {
            this->Draw ();
            this->CurrItem = 0;
            locktype = D3DLOCK_DISCARD | D3DLOCK_NOSYSLOCK;
        }

        hr = this->Buffer->Lock (
            this->CurrItem * this->ItemSize,
            numitems * this->ItemSize,
            (void **) &lockdata,
            locktype
        );

        assert (SUCCEEDED (hr));
        assert (SUCCEEDED (lockdata != NULL));

        memcpy (lockdata, itemdata, numitems * this->ItemSize);

        hr = this->Buffer->Unlock ();
        assert (SUCCEEDED (hr));

        // you may wish to draw here or you may not; up to you

        // advance the pointer
        this->CurrItem += numitems;
    }

    void Draw (void)
    {
        // fill this in yourself
    }

private:
    IDirect3DDevice9 *Device;
    IDirect3DVertexBuffer9 *Buffer;

    int CurrItem;
    int MaxItems;
    int ItemSize;
};