Why is 8th gen GTA V poorly optimized?

On Xbox one/ps4GTAO lags so much and in story mode it's 30 FPS and it's not being a big upgrade in graphics it's a small one also it. Was 4-5 times bigger than og ones
Like PS3 was 16gb Xbox one was 60-80gb Big difference yes all of us are on newgen/pc but 8th gen wasn't optimized like holy Rockstar wasn't even trying like watchdog had better optimization for 8th gen rockstar should fix their ports