Agreed, it’s under 90 minutes, establishes the premise quickly, and then spends the time seeing what can be done within the limitations (as text, the characters are also testing the premise’s limits and trying to divine patterns throughout). Less horror-ish as it goes
IT ENDS: Really good film. And strongly recommended even if BACKROOMS bounced off you for any reason. Totally committed to following its elementally simple premise all the way down the line. (So to speak.) And unlike a lot of contemporary horror, not an iota of cruelty/condescension.