diff --git a/.idea/inspectionProfiles/Project_Default.xml b/.idea/inspectionProfiles/Project_Default.xml
new file mode 100644
index 00000000..d606a954
--- /dev/null
+++ b/.idea/inspectionProfiles/Project_Default.xml
@@ -0,0 +1,6 @@
+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="AndroidLintNewerVersionAvailable" enabled="true" level="WARNING" enabled_by_default="true" />
+  </profile>
+</component>
\ No newline at end of file
diff --git a/README.md b/README.md
index 27356a0e..8a2bd452 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 *Pupil, Hitomi.la viewer for Android*  
 
 ![](https://img.shields.io/github/downloads/tom5079/Pupil/total)  
-[![](https://img.shields.io/github/downloads/tom5079/Pupil/5.1.5-hotfix1/Pupil-v5.1.5-hotfix1.apk?color=%234fc3f7&label=DOWNLOAD%20APP&style=for-the-badge)](https://github.com/tom5079/Pupil/releases/download/5.1.5/Pupil-v5.1.5-hotfix1.apk)  
+[![](https://img.shields.io/github/downloads/tom5079/Pupil/5.1.5-hotfix2/Pupil-v5.1.5-hotfix2.apk?color=%234fc3f7&label=DOWNLOAD%20APP&style=for-the-badge)](https://github.com/tom5079/Pupil/releases/download/5.1.5-hotfix2/Pupil-v5.1.5-hotfix2.apk)
 [![](https://discordapp.com/api/guilds/610452916612104194/embed.png?style=banner2)](https://discord.gg/Stj4b5v)  
 
 # Features
diff --git a/app/build.gradle b/app/build.gradle
index b85c0462..79c232db 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -38,7 +38,7 @@ android {
         minSdkVersion 16
         targetSdkVersion 30
         versionCode 63
-        versionName "5.1.5-hotfix1"
+        versionName "5.1.5-hotfix2"
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         vectorDrawables.useSupportLibrary = true
     }
@@ -77,8 +77,8 @@ android {
 dependencies {
     implementation fileTree(dir: "libs", include: ["*.jar", "*.aar"])
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
-    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.9"
-    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.0.0-RC2"
+    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.4.0-M1"
+    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.0.0"
     
     implementation "androidx.appcompat:appcompat:1.2.0"
     implementation "androidx.activity:activity-ktx:1.2.0-beta01"
diff --git a/app/release/output-metadata.json b/app/release/output-metadata.json
index 1000187d..b21cb189 100644
--- a/app/release/output-metadata.json
+++ b/app/release/output-metadata.json
@@ -11,7 +11,7 @@
       "type": "SINGLE",
       "filters": [],
       "versionCode": 63,
-      "versionName": "5.1.5-hotfix1",
+      "versionName": "5.1.5-hotfix2",
       "outputFile": "app-release.apk"
     }
   ]
diff --git a/app/src/main/java/xyz/quaver/pupil/Pupil.kt b/app/src/main/java/xyz/quaver/pupil/Pupil.kt
index 314dc9fb..0faec406 100644
--- a/app/src/main/java/xyz/quaver/pupil/Pupil.kt
+++ b/app/src/main/java/xyz/quaver/pupil/Pupil.kt
@@ -18,7 +18,10 @@
 
 package xyz.quaver.pupil
 
-import android.app.*
+import android.app.Application
+import android.app.Notification
+import android.app.NotificationChannel
+import android.app.NotificationManager
 import android.content.Context
 import android.content.Intent
 import android.net.Uri
diff --git a/app/src/main/java/xyz/quaver/pupil/ui/view/TagChipGroup.kt b/app/src/main/java/xyz/quaver/pupil/ui/view/TagChipGroup.kt
index c0004260..cb74eb77 100644
--- a/app/src/main/java/xyz/quaver/pupil/ui/view/TagChipGroup.kt
+++ b/app/src/main/java/xyz/quaver/pupil/ui/view/TagChipGroup.kt
@@ -24,10 +24,7 @@ import android.util.AttributeSet
 import android.util.Log
 import com.google.android.material.chip.Chip
 import com.google.android.material.chip.ChipGroup
-import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.async
-import kotlinx.coroutines.launch
+import kotlinx.coroutines.*
 import xyz.quaver.pupil.R
 import xyz.quaver.pupil.types.Tag
 import xyz.quaver.pupil.types.Tags
@@ -71,10 +68,12 @@ class TagChipGroup @JvmOverloads constructor(context: Context, attr: AttributeSe
         maxChipSize = attr.getInt(R.styleable.TagChipGroup_maxTag, Defaults.maxChipSize)
     }
 
+    private var refreshJob: Job? = null
     fun refresh() {
+        refreshJob?.cancel()
         this.removeAllViews()
 
-        CoroutineScope(Dispatchers.Main).launch {
+        refreshJob = CoroutineScope(Dispatchers.Main).launch {
             tags.take(maxChipSize).map {
                 CoroutineScope(Dispatchers.Default).async {
                     TagChip(context, it).apply {
diff --git a/app/src/main/java/xyz/quaver/pupil/util/SavedSet.kt b/app/src/main/java/xyz/quaver/pupil/util/SavedSet.kt
index 24c71f2a..6e2c0987 100644
--- a/app/src/main/java/xyz/quaver/pupil/util/SavedSet.kt
+++ b/app/src/main/java/xyz/quaver/pupil/util/SavedSet.kt
@@ -24,7 +24,7 @@ import kotlinx.serialization.json.Json
 import java.io.File
 import java.util.*
 
-class SavedSet <T: Any> (private val file: File, private val any: T, private val set: MutableSet<T> = Collections.synchronizedSet(mutableSetOf())) : MutableSet<T> by set {
+class SavedSet <T: Any> (private val file: File, private val any: T, private val set: MutableSet<T> = mutableSetOf()) : MutableSet<T> by set {
 
     @Suppress("UNCHECKED_CAST")
     @OptIn(ExperimentalSerializationApi::class)
@@ -39,24 +39,23 @@ class SavedSet <T: Any> (private val file: File, private val any: T, private val
         load()
     }
 
+    @Synchronized
     fun load() {
-        synchronized(this) {
-            set.clear()
-            kotlin.runCatching {
-                Json.decodeFromString(serializer, file.readText())
-            }.onSuccess {
-                set.addAll(it)
-            }
+        set.clear()
+        kotlin.runCatching {
+            Json.decodeFromString(serializer, file.readText())
+        }.onSuccess {
+            set.addAll(it)
         }
     }
 
+    @Synchronized
     @OptIn(ExperimentalSerializationApi::class)
     fun save() {
-        synchronized(this) {
-            file.writeText(Json.encodeToString(serializer, set.toList()))
-        }
+        file.writeText(Json.encodeToString(serializer, set.toList()))
     }
 
+    @Synchronized
     override fun add(element: T): Boolean {
         load()
 
@@ -67,6 +66,7 @@ class SavedSet <T: Any> (private val file: File, private val any: T, private val
         }
     }
 
+    @Synchronized
     override fun addAll(elements: Collection<T>): Boolean {
         load()
 
@@ -77,6 +77,7 @@ class SavedSet <T: Any> (private val file: File, private val any: T, private val
         }
     }
 
+    @Synchronized
     override fun remove(element: T): Boolean {
         load()
 
@@ -85,6 +86,7 @@ class SavedSet <T: Any> (private val file: File, private val any: T, private val
         }
     }
 
+    @Synchronized
     override fun clear() {
         set.clear()
         save()
diff --git a/app/src/main/java/xyz/quaver/pupil/util/file.kt b/app/src/main/java/xyz/quaver/pupil/util/file.kt
index 159c0477..d889b3a1 100644
--- a/app/src/main/java/xyz/quaver/pupil/util/file.kt
+++ b/app/src/main/java/xyz/quaver/pupil/util/file.kt
@@ -55,10 +55,12 @@ fun cleanCache(context: Context) = CoroutineScope(Dispatchers.IO).launch {
             while (cacheSize.invoke() > limit/2) {
                 val caches = cacheFolder.list() ?: return@withLock
 
-                (histories.toList().firstOrNull {
-                    caches.contains(it.toString()) && !downloadManager.isDownloading(it)
-                } ?: return@withLock).let {
-                    Cache.delete(context, it)
+                synchronized(histories) {
+                    (histories.firstOrNull {
+                        caches.contains(it.toString()) && !downloadManager.isDownloading(it)
+                    } ?: return@withLock).let {
+                        Cache.delete(context, it)
+                    }
                 }
             }
     }
diff --git a/app/src/main/java/xyz/quaver/pupil/util/translation.kt b/app/src/main/java/xyz/quaver/pupil/util/translation.kt
index 7f68d607..18f8235e 100644
--- a/app/src/main/java/xyz/quaver/pupil/util/translation.kt
+++ b/app/src/main/java/xyz/quaver/pupil/util/translation.kt
@@ -42,11 +42,13 @@ var translations: Map<String, String> = run {
 @Suppress("BlockingMethodInNonBlockingContext")
 fun updateTranslations() = CoroutineScope(Dispatchers.IO).launch {
     translations = emptyMap()
-    translations = Json.decodeFromString<Map<String, String>>(client.newCall(
-        Request.Builder()
-            .url(contentURL + "${Preferences["tag_language", ""]}.json")
-            .build()
-    ).execute().also { if (it.code() != 200) return@launch }.body()?.use { it.string() } ?: return@launch).filterValues { it.isNotEmpty() }
+    kotlin.runCatching {
+        translations = Json.decodeFromString<Map<String, String>>(client.newCall(
+            Request.Builder()
+                .url(contentURL + "${Preferences["tag_language", ""]}.json")
+                .build()
+        ).execute().also { if (it.code() != 200) return@launch }.body()?.use { it.string() } ?: return@launch).filterValues { it.isNotEmpty() }
+    }
 }
 
 fun getAvailableLanguages(): List<String> {
diff --git a/app/src/main/res/values-ja/strings.xml b/app/src/main/res/values-ja/strings.xml
index 24635c8e..653f6341 100644
--- a/app/src/main/res/values-ja/strings.xml
+++ b/app/src/main/res/values-ja/strings.xml
@@ -154,4 +154,5 @@
     <string name="settings_cache_unlimited">制限なし</string>
     <string name="settings_tag_language">タグ言語</string>
     <string name="settings_tag_language_message">Githubにて翻訳に参加できます</string>
+    <string name="settings_concurrent_download">並列ダウンロード</string>
 </resources>
\ No newline at end of file
diff --git a/app/src/main/res/values-ko/strings.xml b/app/src/main/res/values-ko/strings.xml
index a2ef5add..3dec5094 100644
--- a/app/src/main/res/values-ko/strings.xml
+++ b/app/src/main/res/values-ko/strings.xml
@@ -154,4 +154,5 @@
     <string name="settings_cache_unlimited">무제한</string>
     <string name="settings_tag_language">태그 언어</string>
     <string name="settings_tag_language_message">Github에서 번역에 참여하세요</string>
+    <string name="settings_concurrent_download">병렬 다운로드</string>
 </resources>
\ No newline at end of file
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index a9fc624a..d20929e1 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -176,6 +176,7 @@
 
     <string name="settings_miscellaneous_title">Miscellaneous</string>
     <string name="settings_tag_language">Tag Language</string>
+    <string name="settings_concurrent_download">Concurrent Download</string>
     <string name="settings_tag_language_message">Participate in translation on Github</string>
     <string name="settings_mirror_summary">Load images from mirrors</string>
     <string name="settings_proxy_title">Proxy</string>