Concern around growing complexity in engine - hash table specifically

All, I spent a little bit of time today trying to debug an issue with 7 that Drupal 8 was facing, specifically regarding an array index not behaving correctly ($array["key"] returned null, even though the key existed in the hash table). I noticed that the hash table implementation has gotten orders of magnitude more complex in recent times (since phpng was merged). Specifically, that ardata and arhash are now the same block of memory, and that we're now doing negative indexing into arData to get the hash map list. From Dmitry's commit message, it was done to keep the data that's accessed most often in the same CPU cache line. While I am sure that there are definitive performance gains to doing this, I do worry about the development and debugging costs of this added complexity. As well as the way it increases the busfactor of the project. There is definitely a tradeoff there, as the change is pretty well encapsulated behind macros. But that introduces a new level of abstraction. But deeper than that it really makes debugging with gdb a pain in the neck. Without hard data on this particular patch, I'm not suggesting we roll back the change or anything. I more just want to express concern with the trend lately to increase complexity significantly on developers for the sake of performance. While I'm definitely not saying performance doesn't matter, I also think performance at all costs is dangerous. And I wonder if some of the more fundamental (even if isolated) changes such as this should be way more documented and include the performance justification for them. I'm definitely not suggesting an RFC, but perhaps some level of discussion should be required for these sorts of changes... Thoughts?