Rule 3 was true 1989, back then computers were so slow and had barely any ram so most things you did only was reasonable for small number of inputs. Today we almost always have large amounts of inputs so its different.
It is important to remember that the art of sw engineering (like all engineering) lives in a balance between all these different requirements; not just in OPTIMIZE BIG-O.
At 99% of shops it should be the other way around .
I've personally found it useful to always have concrete numbers in mind. An algorithm or data structure designed for N will probably be fine for N/10 and 10N, but it will often be inefficient for N/1000 or 1000N.