He'd made a Atari ST extension called (I think) Quick ST. The ST had awfully slow text output due to many things such as the interleaved bitmaps and lack of hardware assist. His made it much faster. I wrote a competitor that was faster than his. A couple of month later he wrote a new version that obliterated mine.
I literally spent weeks trying to work out how he'd done it, went through the 68k reference manual, nothing.
Then one day it came to me in an actual dream - 12 year old me actually woke up and started typing. It was the 'move.p' instruction. From memory this instruction split a 16 bit word into 2 bytes but aligned on a word (16 bit) boundary. I added this and my code then beat his.
Bear in mind this was before the internet, so no to-and-fro with conversation, all I saw was what he did. It absolutely stood me in good stead for the rest of my career, each time now I see a programmer who writes something completely sloppy I ask them how it will scale. I really miss those days and wish somebody could bring this back for newer people to learn the basics.