Once you’ve had the PIN scramble turned on for a while, it becomes second nature. I enter my GrapheneOS scrambled PIN about as quickly as the unscrambled PIN on my non‐GrapheneOS work phone. But it’s more of a defense against figuring out my PIN from the finger marks on my screen.
In environments where shoulder surfing is a concern, I prefer to use the multiple profiles feature: log out of my main profile (which is actually a secondary profile) to completely evict its keys from memory, and switch to a burner secondary profile containing no personal data, which unlocks with my fingerprint for convenience.
Second nature or not, I’m not sure how this protects you against the security camera watching you enter the passcode. I guess you’re hoping it can’t read the digit on each key?