[bitL]

Thanks for the radix tree reference!

On the other hand, stating "they could still maintain similar performance" for immutable dictionaries is misleading at best (you are right for dictionary sizes approaching 0). Anyone can look up benchmarks on the Internet showing real-world performance of immutable data structures and make their mind up if they actually can deliver in their particular use cases.

Mutability could be always used to boost performance on Von Neumann architecture if you know what you are doing. It's just becoming impractical due to increased complexity, pervasive security paranoia and the fact that there are very few people that can get things done right. If you want to contribute to advance your case, please create a competing computer architecture as fast as Von Neumann's, physically possible, but based on immutability.

[semi-extrinsic]

> Mutability could be always used to boost performance (...)

For the extreme case, see the EQUIVALENCE statement in old Fortran.