The issue is it's a moving target. What was expensive yesterday could be fast today based on compiler optimizations (and potentially vice versa).

Further, changes in the ISA can open up gains in performance that weren't available in yesteryear. An example of this would be SIMD instruction usage.

It's not a bad idea to know enough assembly language to understand why code is slow. However, the general default should be to avoid writing it. Your time would be better spent getting a newer version of your compiler and potentially enabling things like PGO.