Board Thread:Questions and Answers/@comment-30262883-20161018173139/@comment-27701762-20161019000341

As you kill enemies in an area, the number of steps required until the next encounter becomes larger and larger (plus the random factor). So as you kill more enemies, it takes longer and longer to get to the next fight.

A couple things can help make things go a bit faster. First, obviously, make sure you are in an area where enemies spawn (encounters don't occur on every screen). After that, make sure that you enter the room "fresh" (i.e. move from the room to another room, then go back to reset the step counter). Then hold Up+Down to wallhump, which makes the step counter increase at a faster pace (don't move left or right while doing this). You may still need to wait a while, as the Genocide Route is more about patience. But you should eventually get the encounters.