First of all, redis is amazing, and your 4 month development process speaks to the fact that you've already designed and verified correctness super thoroughly.
... just speaking as someone who sometimes has to review very long PRs sometimes, though, I feel like 25% is a roughly normal level of "signal to noise." 5,000 lines of core logic is a LOT, and the tests and dependencies do still need to be read.
EDIT: I feel like the problem, as a reviewer, is processing 4 months of intensive research/development and providing useful feedback. At that point, there's probably not much major input you can have into the core architecture or strategy, so you're probably not providing much more than a bugbot at that point.
I think where we went wrong in understanding this PR is in the assumption that it's designed to invite review because that's how a lot of other team- or community-driven projects work.
> At that point, there's probably not much major input you can have into the core architecture or strategy
Sure you can? In this concrete case, Redis is very "flat" — there's the data structure implementations, and there's the commands that use them. 1+N. You could have feedback about the data structure (i.e. whether it's optimal for the use-cases); or about any of the commands (i.e. not just their impls, but also whether they're the best core API surface to lock in long-term, or even whether they're worth including at all.)
Any given feedback would necessitate fairly limited rework to address, as you're either modifying the data structure (and its tests) or a command (and its tests and docs.)