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> They have completely different semantics, performance characteristics

Many times it is the different semantics that allow for better performance.

If you can add new methods or change their implementation at runtime, performance optimizations get a lot trickier, for example.

The immutability that's foundational to Elixir and Erlang enable the incredible scaling capabilities of the Erlang VM.

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> The immutability that's foundational to Elixir and Erlang enable the incredible scaling capabilities of the Erlang VM

This. If you want at least one order of magnitude of scaling to come "for free" (note: further orders of magnitude may still cost you), the BEAM VM will hook you up thanks to this fundamental design decision

> If you can add new methods or change their implementation at runtime, performance optimizations get a lot trickier

This particular example is a non-issue though - it's completely solved through dynamic deoptimisation (as in the machine code with and without method redefinition is exactly the same.)

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