GetTextExtentPoint32 is really slow on Windows Mobile, any alternative?

[LifeIsSuffering]

I wrote a simple demo below to discover why GetTextExtentPoint32 is slow,

void CSmartGetTextExtentDlg::OnBnClickedButton1()
{
    // TODO: Add your control notification handler code here
    CDC* dc = GetDC();
    HDC hdc = dc->GetSafeHdc();

    printf("begin GetTextExtent test\n");

    string text1("Initialize the shell activate info structure");
    SIZE size;
    UINT64 startTime = getTicks();
    ::GetTextExtentPoint32(hdc, (LPCTSTR)(text1.c_str()), text1.size(), &size);
    UINT64 endTime = getTicks();
    UINT64 duration = endTime - startTime;
    printf("str1: %d ms\n", duration);

    string text2("Initialize the shell activate info structure");
    startTime = getTicks();
    ::GetTextExtentPoint32(hdc, (LPCTSTR)(text2.c_str()), text2.size(), &size);
    endTime = getTicks();
    duration = endTime - startTime;
    printf("str2: %d ms\n", duration);

    printf("end GetTextExtent test\n\n");

    ReleaseDC(dc);
}

The result was really confusing.

In first run, the 2 resulting duration were all around 300~400ms, even if they were same strings.

Then I clicked button to trigger test again, the time cost was obviously decreased to almost 0ms.

I restarted demo, the result was 0~1ms every time.

Then I reboot my device (not simulator), ran test twice or more like above, the first run was still slow, and became faster after that.

I guessed the system cached something I didn't figure out, but the confusing thing was that why it was slow even if they're 2 same strings in first run.

Any idea about this weird thing and any alternative?

thx in advance.