diff --git a/OpenRA.Mods.Cnc/UtilityCommands/LegacyTilesetImporter.cs b/OpenRA.Mods.Cnc/UtilityCommands/LegacyTilesetImporter.cs
index efd3391958..f9d9dc5dbe 100644
--- a/OpenRA.Mods.Cnc/UtilityCommands/LegacyTilesetImporter.cs
+++ b/OpenRA.Mods.Cnc/UtilityCommands/LegacyTilesetImporter.cs
@@ -168,8 +168,7 @@ namespace OpenRA.Mods.Cnc.UtilityCommands
 			metadata.AppendLine();
 
 			metadata.AppendLine("Terrain:");
-			terrainTypes = terrainTypes.Distinct().ToArray();
-			foreach (var terrainType in terrainTypes)
+			foreach (var terrainType in terrainTypes.Distinct())
 			{
 				metadata.AppendLine($"\tTerrainType@{terrainType}:");
 				metadata.AppendLine($"\t\tType: {terrainType}");
diff --git a/OpenRA.Mods.Common/Scripting/Properties/ProductionProperties.cs b/OpenRA.Mods.Common/Scripting/Properties/ProductionProperties.cs
index 9ff58430cf..42c0777550 100644
--- a/OpenRA.Mods.Common/Scripting/Properties/ProductionProperties.cs
+++ b/OpenRA.Mods.Common/Scripting/Properties/ProductionProperties.cs
@@ -244,8 +244,8 @@ namespace OpenRA.Mods.Common.Scripting
 			foreach (var actorType in actorTypes.Distinct())
 				typeToQueueMap.Add(actorType, GetBuildableInfo(actorType).Queue.First());
 
-			var queueTypes = typeToQueueMap.Values.Distinct().ToList();
-
+			// PERF: queues tend to live for a long time so cast to array.
+			var queueTypes = typeToQueueMap.Values.Distinct().ToArray();
 			if (queueTypes.Any(t => !queues.ContainsKey(t) || productionHandlers.ContainsKey(t)))
 				return false;
 
diff --git a/OpenRA.Mods.Common/Traits/SupportPowers/GrantExternalConditionPower.cs b/OpenRA.Mods.Common/Traits/SupportPowers/GrantExternalConditionPower.cs
index d8bf57bd68..bc2cc971a9 100644
--- a/OpenRA.Mods.Common/Traits/SupportPowers/GrantExternalConditionPower.cs
+++ b/OpenRA.Mods.Common/Traits/SupportPowers/GrantExternalConditionPower.cs
@@ -96,11 +96,12 @@ namespace OpenRA.Mods.Common.Traits
 		public IEnumerable<Actor> UnitsInRange(CPos xy)
 		{
 			var tiles = CellsMatching(xy, footprint, info.Dimensions);
-			var units = new List<Actor>();
+			var units = new HashSet<Actor>();
 			foreach (var t in tiles)
-				units.AddRange(Self.World.ActorMap.GetActorsAt(t));
+				foreach (var a in Self.World.ActorMap.GetActorsAt(t))
+					units.Add(a);
 
-			return units.Distinct().Where(a =>
+			return units.Where(a =>
 			{
 				if (!info.ValidRelationships.HasRelationship(Self.Owner.RelationshipWith(a.Owner)))
 					return false;
diff --git a/OpenRA.Mods.Common/UtilityCommands/ExtractEmmyLuaAPI.cs b/OpenRA.Mods.Common/UtilityCommands/ExtractEmmyLuaAPI.cs
index f438942474..e6e35ac7e0 100644
--- a/OpenRA.Mods.Common/UtilityCommands/ExtractEmmyLuaAPI.cs
+++ b/OpenRA.Mods.Common/UtilityCommands/ExtractEmmyLuaAPI.cs
@@ -128,7 +128,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 			Console.WriteLine("---A list of ActorInit implementations that can be used by Lua scripts.");
 			Console.WriteLine("---@class initTable");
 
-			var localEnums = new List<Type>();
+			var localEnums = new HashSet<Type>();
 			foreach (var init in actorInits)
 			{
 				var name = init.Name[..^4];
@@ -159,7 +159,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 				}
 			}
 
-			usedEnums = localEnums.Distinct();
+			usedEnums = localEnums;
 		}
 
 		static void WriteEnums(IEnumerable<Type> enumTypes)
diff --git a/OpenRA.Mods.Common/Widgets/Logic/Installation/ModContentLogic.cs b/OpenRA.Mods.Common/Widgets/Logic/Installation/ModContentLogic.cs
index 9fe0f02bef..ddf6ecb405 100644
--- a/OpenRA.Mods.Common/Widgets/Logic/Installation/ModContentLogic.cs
+++ b/OpenRA.Mods.Common/Widgets/Logic/Installation/ModContentLogic.cs
@@ -112,9 +112,8 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 				requiredWidget.IsVisible = () => p.Value.Required;
 
 				var sourceWidget = container.Get<ImageWidget>("SOURCE");
-				var sourceTitles = p.Value.Sources.Select(s => sources[s].Title).Distinct().ToList();
-				var sourceList = sourceTitles.JoinWith("\n");
-				var isSourceAvailable = sourceTitles.Count != 0;
+				var sourceList = p.Value.Sources.Select(s => sources[s].Title).Distinct().JoinWith("\n");
+				var isSourceAvailable = sourceList.Length != 0;
 				sourceWidget.GetTooltipText = () => sourceList;
 				sourceWidget.IsVisible = () => isSourceAvailable;
 
diff --git a/OpenRA.Mods.D2k/Activities/SwallowActor.cs b/OpenRA.Mods.D2k/Activities/SwallowActor.cs
index e0980a2bf2..fee76d9aca 100644
--- a/OpenRA.Mods.D2k/Activities/SwallowActor.cs
+++ b/OpenRA.Mods.D2k/Activities/SwallowActor.cs
@@ -76,7 +76,7 @@ namespace OpenRA.Mods.D2k.Activities
 			positionable.SetPosition(self, targetLocation);
 
 			var attackPosition = self.CenterPosition;
-			var affectedPlayers = targets.Select(x => x.Owner).Distinct().ToList();
+			var affectedPlayers = targets.Select(x => x.Owner).ToHashSet();
 			Game.Sound.Play(SoundType.World, swallow.Info.WormAttackSound, self.CenterPosition);
 
 			foreach (var player in affectedPlayers)