RTIC throttle register is to define how many SEC engine clocks to wait to launch the next descriptor to hash the next block of data. This is used to keep RTIC from using too much bus bandwidth.
So it depends on your use case and whether you are using the SEC engine for other operations (e.g. IPsec, VPN, TLS, etc).
I will suggest initially set RTIC Throttle (RTHR) at 0x176001c to 0xff000000, and adjust the value accordingly to your use case.