Each level in Doom has an associated par time. After completing a level, the par time is displayed on the intermission screen next to the the time that the player took to complete the level. This allows the player to gauge their skill by comparing their time.
Doom par time

DooM 2 Map01 par time

The par times are encoded into the Doom executable in the file g_game.c. John Romero generated the par times by completing each level as fast as possible, rounding off the resulting time and adding 30 seconds.

Par times

The following are the par times from the Doom source code:


Doom II