Killough notably added many performance optimizations to the Doom engine. The single most important efficiency improvement was replacing the linear search used for looking up game data resources with a hash table algorithm. This improvement sped up the engine over 300%.
Between 1997 and 1998, he also operated a website with extensive technical information about Doom. In 2002, John Romero put up an archived copy of Killough's web pages on his own website. Romero calls the website "legendary" and writes that for many years Killough "was The Man to go to for any DOOM technical trivia".
In his regular job, he now tunes mathematical libraries such as BLAS to run fast on supercomputers. For several years, Killough also maintained a page on priority queues.
- Figure from A Slightly Condensed Genealogy of Doom Source Ports (Doomworld article)
- Quote from the General Doom information page on John Romero's website (archived)