Is that actually a high review rate? Especially if you know the language and domain. Sure, initially there's a learning curve for a new codebase structure, but lots of lines will also be trivial and many changes might also be similar to each other.
But then you need to watch for bugs coming from interaction with previous changes and in 700k loc that might be nontrivial. How do you know which states are reachable and which are not? That takes time.
It only takes a botched condition here (forgot a "not"? swapped "and"/"or"?), a swapped variable name there, code that looks ok, but isn't.