Yes, GRDB even w/ manual help is obviously not as fast raw SQLite. As much respect I have for the author "performance nearly identical as raw SQLite" is incorrect. Lighter also achieves some of the performance characteristics by avoiding allocations for bound parameters (such being statically generated).
I didn't look into SharingGRDB yet, but it seems like those macros could accomplish similar performance, the other way around (Lighter works on the SQLite DB schema).
What I'm not entirely sure of yet is why it even sits on top of GRDB in the first place, instead of just doing the SQLite parts itself. Marketing I suppose.