The first performance problem I've found and fixed in code written by someone else was in Gadu-Gadu. GG was an Windows instant messenger very popular in Poland in the early 2000s. It was closed, binary-only code but due to its popularity a modding "underground" has been formed quickly and soon, its protocol was reverse engineered.
I was about 18 years old at the time and I had a running competition with a friend to make our Windows systems look and feel like Amiga OS. We had the basic things covered, but some programs were not themeable at the time. GG was one of them and I thought: challenge accepted.
GG always had this annoying bug that the longer your conversation was the slower the program was. And boy, our conversations were long these days, we were teens and had lots of time on our hands. Sometimes we talked so much that the program froze for several seconds after I pressed Enter.
If you want to verify my story. the PowerGG plugin was briefly named "Amigo" and then released as "Foolteen".