GrapheneOS zeroes memory at shutdown/reboot and again at boot time. It also currently ends up zeroing most of the memory twice at boot because enabling zero-on-free causes the upstream code to partially zero memory. That takes extra time but is very important for security against physical attacks. It can eventually be dropped once there's a hardware feature providing a per-boot encryption key for all of the RAM. GrapheneOS also extends verified boot to system APK updates and rebuilds the APK metadata cache at boot as part of this. These features are very important to improve security and don't add more than a few seconds of time to boot.
The verified boot notice screen can be skipped by double tapping power to pause and unpause as mentioned earlier in the thread to save more time than the security features we add to the boot / shutdown process add to a reboot.