The GrapheneOS team is security focused to the point where it is detrimental to the regular user experience. I.e. "Secure App Spawning" increases app startup time considerably on older devices like the Pixel 4a.

That's why Graphene allows you to disable the security features. Turning off secure app spawning won't make your device incredibly vulnerable, it will just be set back to normal AOSP security level.

Also, the GrapheneOS team has very high standards for security features supported by a phone. Basically no phone besides Pixel supports those features, which obviously isn't a big problem for most people (else we'd have a big problem).

You know which phone has basically all of those security features? The iPhone. GrapheneOS is not building something insane, they're just hardening Android to a point where it's actually comparable to iPhone security. Sure, usability might not be perfect because Google only releases base Android as open source software and keeps all their fancy apps proprietary, but it's not in a state where it's totally unusable either.