[rainsford]

One underappreciated thing about ChaCha that I think a lot of people miss is that it's not just that the operations are simple, but DJB clearly spent almost as much time approaching the design as a computer engineer as he did as a cryptographer. He obviously thought about pipelining in modern processors, available registers, cache size, implementation details that made some operations faster with specific inputs, etc. It's unlikely a generic ARX construction could achieve the same security level with the same speed without similar careful consideration of non-cryptographic factors.