"optimize for instruction count" makes sense in a universe where a megabyte is a huge (and hugely expensive) amount of RAM. Especially on a machine with virtual memory* and poor page swapping algorithms. A program that exceeded it's available RAM by even a few bytes could take a horrible hit in performance due to disk swapping. A less than speed optimal program may well outperform a speed optimized program if stays resident in RAM.


*) Virtual memory in it's original sense, not today's definition which is mostly just memory management.