according to 3GPP TS 24.011:
“the MS shall transmit a CM SERVICE REQUEST for the new CM connection before the final CP‑ACK (e.g. the one that acknowledges the CP‑DATA that carried the RP‑ACK) for the old MM connection is transmitted”
There is delay between CM Service Request (CMSREQ) and CP Acknowledgement (CPACK)
When using Nokia mobile, it is fine and there is no delay, the delay is there when using Siemens mobile.