This is an average rate. There are large parts of the code base that are configurations, mappings, or just glue code. Even if you're not familliar with the language you get a sense for it quickly, you can power through those at 10kloc or higher when you hit a run, you can also rule out directories quickly after you see an example. You can also ask the people you're reviewing "is this directory just all kube config? where's the security settings?". and go at a high rate for reviewing the code.

When you get to interesting code you then slow way down, maybe 10loc / minute.

Remember, for diligence, you're doing risk reduction, your job specifically is determine that there is in fact secret sauce, the product is there, not to understand it. In fact it's problematic if you 100% understand that secret sauce.