I'm of the opinion that the NSA must have a long running covert program to discourage the adoption of secure operating systems.
Multilevel Secure Operating Systems have existed since the 1980s, yet most people haven't even heard of them.
Except for the Xbox. Someone hacking an Xbox would lose _Microsoft_ money through piracy, which hurts their bottom line!
As a result, the Xbox runs a type 1 hypervisor with various compiler hardening options like Control Flow Integrity enabled, and are generally some of the most locked down consumer devices sold today.
What's Apple's motivation beyond bad press? iMessage was parsing media sent to it with a pdf parser. Sure, some activists in repressive regimes will get spied on and executed, but it's just so much effort to invest dev effort on rewrites for security when you could find new ways to send cute animated gifs to each other.
From what I can tell all NSO's rigmarole of making a virtual machine in the PDF parser is to work around the existing mitigations.
I guess they could also turn on asan etc in production but that's more than a few percent slowdown.
The whole OS feels much slower than regular Google Android, Osmand (map app) is barely usable (on a Pixel 4XL which isn't a low end phone by any means).
So I don't think we can discount how much slower a device will be with a more secure OS. It might be invisible on desktops, but certainly not on mobile.
Android and iOS are optimized in general and for example Google or Samsung also optimize and distribute it for specific hardware. GrapheneOS is not well funded and has little influence in hardware development. The development of Graphene from AOSP is pretty much guaranteed to de-optimize it in the short term.
While there is little about mobile that would cause a secure OS to be noticably slower, much less mature software with many fewer deployments is expectedly less optimized.
Immunity can't prevent disease from entering your body either; They can only make it harder and actively respond to intrusion.
In contrast, until a few years ago, it was official government policy that EAL4 certification (i.e. "demonstrating resistance to penetration attackers with an Enhanced-Basic attack potential"[5]) was adequate for government systems. This likely corresponds to Class C2 under the old Orange Book standard. This standard was chosen in the interest of allowing standard IT commercial vendors, such as Microsoft and Unix vendors, to submit competitive bids because it was deemed economically and technically infeasible for those systems to be retrofitted with adequate security to achieve a higher standard[6]. So, they instead set the standard to a level achievable by standard commercial IT vendors. They have since reduced it to EAL2 because they determined that requiring a EAL4 certification was too onerous, disqualified large vendors such as FireEye, and was unnecessary as the layering of enough EAL2 systems, each like a piece of swiss cheese, would result in systems comparable to EAL4 systems.
[1] https://www.commoncriteriaportal.org/files/ccfiles/CCPART3V3... Page 34
[2] https://www.niap-ccevs.org/profile/Info.cfm?PPID=65&id=65
[3] https://www.niap-ccevs.org/MMO/PP/pp_skpp_hr_v1.03.pdf Page 84
[4] https://www.commoncriteriaportal.org/files/ccfiles/CCPART3V3... Page 42
[5] https://www.commoncriteriaportal.org/files/ccfiles/CCPART3V3... Page 38
[6] https://www.commoncriteriaportal.org/files/ccfiles/CCPART3V3... Page 38