I rewrote my bosses code one time (she wasn't a full on developer, more an ideas-gal), which implemented a huge speedup, possible > x20. We were doing a exam system that would pull in results for a 5 year course. She had lot of loops calling SQL queries, I replaces it with one big SQL query, that pulled in all the data, and then looped through the results, displaying them correctly. This stopped the page from timing out, which made everyone happy.

Only problem was, when I left someone else had to take over that project, and they didn't understand what I had done either. The original way was the easiest to understand, my way was very messy but fast. I recommended that they look at the original code.