How could they get huge wins out of ignoring the proverbial 80%? Because the minority code had indirect influence on the rest. Changes to the JIT code, nominally responsible for only 20% of CPU time, caused random, outsized performance effects throughout the system.
To understand why, remember that a computer really does only two things: read data and write data. Performance comes down to how much data the computer must move around, and where it goes. Throughput and latency always have the last laugh. This includes CPU instructions, the bits and bytes of the program, which we normally don’t think about.