diff --git a/code/controllers/subsystems/mobs.dm b/code/controllers/subsystems/mobs.dm
index a42caad0d9..4f7d524a8f 100644
--- a/code/controllers/subsystems/mobs.dm
+++ b/code/controllers/subsystems/mobs.dm
@@ -15,24 +15,23 @@ SUBSYSTEM_DEF(mobs)
 	var/list/currentrun = list()
 	var/log_extensively = FALSE
 	var/list/timelog = list()
-	var/list/busy_z_levels = list()
+	
 	var/slept_mobs = 0
+	var/list/process_z = list()
 
 /datum/controller/subsystem/mobs/stat_entry()
 	..("P: [global.mob_list.len] | S: [slept_mobs]")
 
 /datum/controller/subsystem/mobs/fire(resumed = 0)
-	var/list/busy_z_levels = src.busy_z_levels
-
 	if (!resumed)
-		slept_mobs = 0
 		src.currentrun = mob_list.Copy()
-		busy_z_levels.Cut()
-		for(var/played_mob in player_list)
-			if(!played_mob || isobserver(played_mob))
-				continue
-			var/mob/pm = played_mob
-			busy_z_levels |= pm.z
+		process_z.Cut()
+		slept_mobs = 0
+		var/level = 1
+		while(process_z.len < GLOB.living_players_by_zlevel.len)
+			process_z.len++
+			process_z[level] = GLOB.living_players_by_zlevel[level].len
+			level++
 
 	//cache for sanic speed (lists are references anyways)
 	var/list/currentrun = src.currentrun
@@ -41,18 +40,13 @@ SUBSYSTEM_DEF(mobs)
 		var/mob/M = currentrun[currentrun.len]
 		currentrun.len--
 
-		if(QDELETED(M))
+		if(!M || QDELETED(M))
 			mob_list -= M
-		else
-			// Right now mob.Life() is unstable enough I think we need to use a try catch.
-			// Obviously we should try and get rid of this for performance reasons when we can.
-			try
-				if(M.low_priority && !(M.z in busy_z_levels))
-					slept_mobs++
-					continue
-				M.Life(times_fired)
-			catch(var/exception/e)
-				log_runtime(e, M, "Caught by [name] subsystem")
+		else if(M.low_priority && !(process_z[get_z(M)]))
+			slept_mobs++
+			continue
+		
+		M.Life(times_fired)
 
 		if (MC_TICK_CHECK)
 			return