It is the buffer implementation.
[u1 10kTok]->[a1]->[u2]->[a2]. If you branch between the assistant1 and user2 answers then MLX does reprocess the u1 prompt of let's say 10k tokens while llama.cpp does not.
I just tested with GGUF and MLX of Qwen3-Coder-Next with llama.cpp and now with LMStudio. As I do branching very often, it is highly annoying for me to the point of being unusable. Q3-30B is much more usable then on Mac - but by far not as powerful.