Not prohibitively inefficient but fundamentally inefficient. SUBLEQ was designed to be a simple processor that could be implemented and programmed in a course.

Not prohibitively inefficient but fundamentally inefficient. SUBLEQ was designed to be a simple processor that could be implemented and programmed in a course. It was not designed to be practical or efficient, only demonstrative. From a processor design prospective, the problem with SUBLEQ as an instruction set is that its instruction is both an arithmetic operation and a branch. And some things have to be implemented using self-modifying code. These are impediments to branch prediction, instruction pipelining and reordering, and speculation. To make the fastest processor possible for SUBLEQ, one would have to basically have to recompile the code into a better instruction set. And that would be hard as it would require recovering the basic block structure which is hidden by the self-modifying code.