Hi ts01399984,
For boot time optimization, you could refer to the following instruction.
Boot Time Optimization — Jetson Linux Developer Guide documentation (nvidia.com)
Or you could also refer to the FAQ in the following topic to remove 5s boot delay of UEFI:
Jetson AGX Orin FAQ - Jetson & Embedded Systems / Jetson AGX Orin - NVIDIA Developer Forums
Sorry, as you could see the UEFI logs, there’re many boot up stuffs should be done in UEFI.
You could just use r35.3.1 branch in your case.