mxs-auart stop bit-RTS delay

I want to use the AUART0 of the i.MX28 with the handshake signals. For this I am using the latest stable Linux Kernel 3.14.1.

The AUART0 part of the device tree:

	auart0: serial@8006a000 {
   	pinctrl-names = "default";
   	pinctrl-0 = <&auart0_pins_a>; //four pins RX,TX,CTS,RTS
   	fsl,uart-has-rtscts;
   	status = "okay";
    };

So the device tree uses all four pins of this UART. In Linux itself I configure the interface with:

stty -F /dev/ttyAPP0 raw 9600 -echo -echoe -echok -echoctl -echoke crtscts

The UART and the handshake signals are working in principle. But I've noticed a delay between the RTS signal and the stopbit as you can see in the attached figure. If I have understood the RTS/CTS system correctly the RTS signal should be toggled immediately after the stopbit is sent. So that 13 ms (from the logic analyzer image) are really unnecessary and I can not trace it back where they are from. With other baudrates it is no less and no more than the 13 ms (~2...18ms-spread inside of the same baudrate)

I tried this too:

stty -F /dev/ttyAPP0 raw 9600 -echo -echoe -echok -echoctl -echoke -crtscts

The bits CTSEN and RTSEN of the UARTAPP_CTRL2 register are toggled, but there is still a something similar on the RTS lines, but now it is no CTS needed.

So my questions are, where are this delays come from?

How can I be sure who is toggling the RTS line (Linux or the i.MX28 hardware module) ?

image logic analyzer:
https://community.freescale.com/servlet/JiveServlet/download/398158-274040/auart0_rts.png