iOS is probably not nearly as security-hardened as Graphene (or even plain AOSP). iOS relies mostly on "security through obscurity" rather than actual hardening

iOS is probably not nearly as security-hardened as Graphene (or even plain AOSP). iOS relies mostly on "security through obscurity" rather than actual hardening. Apple hopes to maintain security by making the system so complicated and proprietary that it's difficult to understand, let alone find bugs in. Graphene/AOSP hopes to maintain security through real open-source improvements that can be independently verified. It's hard to find bugs there not because access to the sources are unavailable, but because there are usually less bugs in the first place. So, which approach is better? Well, critical exploits are discovered in iOS left and right, so that probably tells you something. Also, I don't think it's fair to assume that Graphene is not playing some role in the slowness (though I don't believe most of the currently-implemented hardening is expected to create dramatic slowdown). Perhaps you should try running the stock ROM for a day or two (with the Google apps disabled in Settings, of course).