It seems like the original GTAV is a efficient and well optimized game. I played it and it looks amazing for how fast it runs. They certainly ran a lot of profilers to nail the performance down. It's amazing what video games can do, look at the Doom series.

https://www.youtube.com/watch?v=UsmqWSZpgJY

But this is online GTA. I can imagine that they handed it off to another department that didn't have the same amount of discipline and expertise. it's probably an off shoot team that creates content for the engine but don't necessarily need to dig down into how the engine works.