I think GP was talking about optimizing away the O(N) call on the last line. The GC will take care of removing the reference to the old (mutable) dict, but constructing a new frozendict from a mutable dict would, in the current proposal, be an O(N) shallow copy.

There are also potentially other optimizations that could be applied (not specific to dict/frozendict) to reduce the memory overhead on operations like "a = f(a)" for selected values of "f".