diff --git a/Lib9c/Battle/StageSimulator.cs b/Lib9c/Battle/StageSimulator.cs
index 6a895fa4bb..0997089d7d 100644
--- a/Lib9c/Battle/StageSimulator.cs
+++ b/Lib9c/Battle/StageSimulator.cs
@@ -257,9 +257,9 @@ private void SetWave(StageSheet.Row stageRow, StageWaveSheet.Row stageWaveRow)
         {
             var enemyStatModifiers = stageRow.EnemyInitialStatModifiers;
             var waves = stageWaveRow.Waves;
-            foreach (var wave in waves
-                         .Select(e => SpawnWave(e, enemyStatModifiers)))
+            foreach (var waveData in waves)
             {
+                var wave = SpawnWave(waveData, enemyStatModifiers);
                 _waves.Add(wave);
             }
         }
diff --git a/Lib9c/Battle/WeightedSelector.cs b/Lib9c/Battle/WeightedSelector.cs
index 1f05675bcb..3549a233c4 100644
--- a/Lib9c/Battle/WeightedSelector.cs
+++ b/Lib9c/Battle/WeightedSelector.cs
@@ -49,13 +49,17 @@ public IEnumerable<T> Select(int count)
             var result = new List<T>();
             var weight = 0m;
             var rnd = _random.Next(1, 100001) * 0.00001m;
-            var sum = _items.Sum(i => i.Weight);
+            decimal sum = 0;
+            foreach (var item in _items)
+            {
+                sum += item.Weight;
+            }
             var ratio = 1.0m / sum;
             while (result.Count < count)
             {
-                foreach (var item in _items.OrderBy(i => i.Weight).ToList())
+                foreach (var item in _items.OrderBy(i => i.Weight))
                 {
-                    weight += (item.Weight * ratio);
+                    weight += item.Weight * ratio;
 
                     if (rnd <= weight)
                     {
diff --git a/Lib9c/Model/Quest/QuestList.cs b/Lib9c/Model/Quest/QuestList.cs
index 1205b0716a..db2e6bf5f1 100644
--- a/Lib9c/Model/Quest/QuestList.cs
+++ b/Lib9c/Model/Quest/QuestList.cs
@@ -235,28 +235,34 @@ public void UpdateTradeQuest(TradeType type, FungibleAssetValue price)
 
         public void UpdateStageQuest(CollectionMap stageMap)
         {
-            var stageQuests = _quests.OfType<WorldQuest>();
-            foreach (var quest in stageQuests)
+            foreach (var quest in _quests)
             {
-                quest.Update(stageMap);
+                if (quest is WorldQuest worldQuest)
+                {
+                    worldQuest.Update(stageMap);
+                }
             }
         }
 
         public void UpdateMonsterQuest(CollectionMap monsterMap)
         {
-            var monsterQuests = _quests.OfType<MonsterQuest>();
-            foreach (var quest in monsterQuests)
+            foreach (var quest in _quests)
             {
-                quest.Update(monsterMap);
+                if (quest is MonsterQuest monsterQuest)
+                {
+                    monsterQuest.Update(monsterMap);
+                }
             }
         }
 
         public void UpdateCollectQuest(CollectionMap itemMap)
         {
-            var collectQuests = _quests.OfType<CollectQuest>();
-            foreach (var quest in collectQuests)
+            foreach (var quest in _quests)
             {
-                quest.Update(itemMap);
+                if (quest is CollectQuest collectQuest)
+                {
+                    collectQuest.Update(itemMap);
+                }
             }
         }
 
@@ -326,10 +332,8 @@ public IValue Serialize()
                 return Dictionary.Empty
                     .SetItem(ListVersionKey, _listVersion.Serialize())
                     .SetItem(QuestsKey, (IValue) new List(_quests
-                        .OrderBy(i => i.Id)
                         .Select(q => q.Serialize())))
                     .SetItem(CompletedQuestIdsKey, (IValue) new List(completedQuestIds
-                        .OrderBy(i => i)
                         .Select(i => i.Serialize())));
             }
 
diff --git a/Lib9c/Model/State/AvatarState.cs b/Lib9c/Model/State/AvatarState.cs
index a9ff513a8a..48267cce17 100644
--- a/Lib9c/Model/State/AvatarState.cs
+++ b/Lib9c/Model/State/AvatarState.cs
@@ -523,18 +523,16 @@ public void UpdateFromQuestReward2(Quest.Quest quest, MaterialItemSheet material
 
         public void UpdateQuestRewards(MaterialItemSheet materialItemSheet)
         {
-            var completedQuests = questList
-                .Where(quest => quest.Complete && !quest.IsPaidInAction)
-                .ToList();
-            // 완료되었지만 보상을 받지 않은 퀘스트를 return 문에서 Select 하지 않고 미리 저장하는 이유는
-            // 지연된 실행에 의해, return 시점에서 이미 모든 퀘스트의 보상 처리가 완료된 상태에서
-            // completed를 호출 시 where문의 predicate가 평가되어 컬렉션이 텅 비기 때문이다.
-            var completedQuestIds = completedQuests.Select(quest => quest.Id).ToList();
-            foreach (var quest in completedQuests)
+            var completedQuestIds = new List<int>();
+            foreach (var quest in questList)
             {
+                if (!quest.Complete || quest.IsPaidInAction)
+                {
+                    continue;
+                }
+                completedQuestIds.Add(quest.Id);
                 UpdateFromQuestReward(quest, materialItemSheet);
             }
-
             questList.completedQuestIds = completedQuestIds;
         }