diff --git a/src/gamedata/g_mapinfo.h b/src/gamedata/g_mapinfo.h
index 887d1c7b2..e96b1c458 100644
--- a/src/gamedata/g_mapinfo.h
+++ b/src/gamedata/g_mapinfo.h
@@ -476,6 +476,7 @@ enum ESkillProperty
 	SKILLP_Infight,
 	SKILLP_PlayerRespawn,
 	SKILLP_SpawnMulti,
+	SKILLP_InstantReaction,
 };
 enum EFSkillProperty	// floating point properties
 {
@@ -520,6 +521,7 @@ struct FSkillInfo
 	double Aggressiveness;
 	int SpawnFilter;
 	bool SpawnMulti;
+	bool InstantReaction;
 	int ACSReturn;
 	FString MenuName;
 	FString PicName;
diff --git a/src/gamedata/g_skill.cpp b/src/gamedata/g_skill.cpp
index 35b8b08d9..34dde60e1 100644
--- a/src/gamedata/g_skill.cpp
+++ b/src/gamedata/g_skill.cpp
@@ -77,6 +77,7 @@ void FMapInfoParser::ParseSkill ()
 	skill.Aggressiveness = 1.;
 	skill.SpawnFilter = 0;
 	skill.SpawnMulti = false;
+	skill.InstantReaction = false;
 	skill.ACSReturn = 0;
 	skill.MustConfirm = false;
 	skill.Shortcut = 0;
@@ -197,6 +198,10 @@ void FMapInfoParser::ParseSkill ()
 		{
 			skill.SpawnMulti = true;
 		}
+		else if (sc.Compare ("InstantReaction"))
+		{
+			skill.InstantReaction = true;
+		}
 		else if (sc.Compare("ACSReturn"))
 		{
 			ParseAssign();
@@ -400,8 +405,12 @@ int G_SkillProperty(ESkillProperty prop)
 
 		case SKILLP_PlayerRespawn:
 			return AllSkills[gameskill].PlayerRespawn;
+
 		case SKILLP_SpawnMulti:
 			return AllSkills[gameskill].SpawnMulti;
+
+		case SKILLP_InstantReaction:
+			return AllSkills[gameskill].InstantReaction;
 		}
 	}
 	return 0;
@@ -540,6 +549,7 @@ FSkillInfo &FSkillInfo::operator=(const FSkillInfo &other)
 	Aggressiveness= other.Aggressiveness;
 	SpawnFilter = other.SpawnFilter;
 	SpawnMulti = other.SpawnMulti;
+	InstantReaction = other.InstantReaction;
 	ACSReturn = other.ACSReturn;
 	MenuName = other.MenuName;
 	PicName = other.PicName;
diff --git a/src/playsim/p_mobj.cpp b/src/playsim/p_mobj.cpp
index 308c58144..68583f8c8 100644
--- a/src/playsim/p_mobj.cpp
+++ b/src/playsim/p_mobj.cpp
@@ -4477,7 +4477,7 @@ void ConstructActor(AActor *actor, const DVector3 &pos, bool SpawningMapThing)
 
 	FRandom &rng = Level->BotInfo.m_Thinking ? pr_botspawnmobj : pr_spawnmobj;
 
-	if (actor->isFast() && actor->flags3 & MF3_ISMONSTER)
+	if (!!G_SkillProperty(SKILLP_InstantReaction) && actor->flags3 & MF3_ISMONSTER)
 		actor->reactiontime = 0;
 
 	if (actor->flags3 & MF3_ISMONSTER)
diff --git a/wadsrc/static/mapinfo/chex.txt b/wadsrc/static/mapinfo/chex.txt
index 24a577b5c..48a49dd0d 100644
--- a/wadsrc/static/mapinfo/chex.txt
+++ b/wadsrc/static/mapinfo/chex.txt
@@ -183,6 +183,7 @@ skill nightmare
 {
 	AmmoFactor = 2
 	FastMonsters
+	InstantReaction
 	DisableCheats
 	RespawnTime = 12
 	SpawnFilter = Nightmare
diff --git a/wadsrc/static/mapinfo/doomcommon.txt b/wadsrc/static/mapinfo/doomcommon.txt
index c4ebda685..5b468c1b6 100644
--- a/wadsrc/static/mapinfo/doomcommon.txt
+++ b/wadsrc/static/mapinfo/doomcommon.txt
@@ -217,6 +217,7 @@ skill nightmare
 {
 	AmmoFactor = 2
 	FastMonsters
+	InstantReaction
 	DisableCheats
 	RespawnTime = 12
 	SpawnFilter = Nightmare
diff --git a/wadsrc/static/mapinfo/heretic.txt b/wadsrc/static/mapinfo/heretic.txt
index 3c8fba839..bdc68c62d 100644
--- a/wadsrc/static/mapinfo/heretic.txt
+++ b/wadsrc/static/mapinfo/heretic.txt
@@ -315,6 +315,7 @@ skill nightmare
 	AmmoFactor = 1.5
 	DoubleAmmoFactor = 1.5
 	FastMonsters
+	InstantReaction
 	DisableCheats
 	SpawnFilter = Nightmare
 	Name = "$MNU_BLACKPLAGUE"
diff --git a/wadsrc/static/mapinfo/hexen.txt b/wadsrc/static/mapinfo/hexen.txt
index 2e41f38ce..da727a6fc 100644
--- a/wadsrc/static/mapinfo/hexen.txt
+++ b/wadsrc/static/mapinfo/hexen.txt
@@ -456,6 +456,7 @@ skill nightmare
 	AmmoFactor = 1.5
 	DoubleAmmoFactor = 1.5
 	FastMonsters
+	InstantReaction
 	DisableCheats
 	SpawnFilter = Nightmare
 	Name = "$MNU_BLACKPLAGUE"
diff --git a/wadsrc/static/zscript/constants.zs b/wadsrc/static/zscript/constants.zs
index 4cfff28a9..4840b50e7 100644
--- a/wadsrc/static/zscript/constants.zs
+++ b/wadsrc/static/zscript/constants.zs
@@ -963,6 +963,7 @@ enum ESkillProperty
 	SKILLP_Infight,
 	SKILLP_PlayerRespawn,
 	SKILLP_SpawnMulti,
+	SKILLP_InstantReaction,
 };
 enum EFSkillProperty	// floating point properties
 {