Code review
People have forgotten what code review is for.
Code review is an asynchronous alternative to pair programming. Like pair programming the goal is that more than one person knows how the code works, understands the design, and it acts as a check-of-last-resort that the design is sound and the code implements the design.
So if you've been sent a multi-thousand line diff to review you're perfectly within your rights to tell the author to book a meeting with you so they can explain the code.
If done well this is much faster than repeatedly round-tripping code reviews in whatever tool you're using.