Improve the Call Setup Time for EPS Fall back 5GSA call

Hi guys, now my CST for EPSFB call is about 6s to 8s
How can I improve that, I have seen the UE’s log trace. My network is set at Release with Redirection without measurement to enhance the CST, but when I saw the log file, the security mode command in TAU procedure wasted too much time (about 300-400ms).
How can I improve that, please help me!!!
Can I improve the CST at SIP protocol??