diff --git a/lib/Compilation.js b/lib/Compilation.js
index 39fa69831..ec1755538 100644
--- a/lib/Compilation.js
+++ b/lib/Compilation.js
@@ -88,13 +88,13 @@ class Compilation extends Tapable {
 	}
 
 	addModule(module, cacheGroup) {
-		cacheGroup = cacheGroup || "m";
 		const identifier = module.identifier();
 		if(this._modules[identifier]) {
 			return false;
 		}
-		if(this.cache && this.cache[cacheGroup + identifier]) {
-			const cacheModule = this.cache[cacheGroup + identifier];
+		const cacheName = (cacheGroup || "m") + identifier;
+		if(this.cache && this.cache[cacheName]) {
+			const cacheModule = this.cache[cacheName];
 
 			let rebuild = true;
 			if(!cacheModule.error && cacheModule.cacheable && this.fileTimestamps && this.contextTimestamps) {
@@ -115,7 +115,7 @@ class Compilation extends Tapable {
 		module.unbuild();
 		this._modules[identifier] = module;
 		if(this.cache) {
-			this.cache[cacheGroup + identifier] = module;
+			this.cache[cacheName] = module;
 		}
 		this.modules.push(module);
 		return true;
@@ -752,7 +752,7 @@ class Compilation extends Tapable {
 				iterationOfArrayCallback(block.dependencies, iteratorDependency);
 			}
 			if(block.blocks) {
-				var blocks = block.blocks.slice();
+				var blocks = block.blocks;
 				var indexBlock = blocks.length;
 				while(indexBlock--) {
 					iteratorBlock(blocks[indexBlock]);
@@ -900,37 +900,50 @@ class Compilation extends Tapable {
 	}
 
 	applyModuleIds() {
-		const unusedIds = [];
-		let nextFreeModuleId = 0;
-		const usedIds = [];
-		const usedIdMap = {};
+		var unusedIds = [];
+		var nextFreeModuleId = 0;
+		var usedIds = [];
+		// TODO
+		var usedIdMap = Object.create(null);
 		if(this.usedModuleIds) {
 			Object.keys(this.usedModuleIds).forEach(key => {
 				const id = this.usedModuleIds[key];
-				if(typeof usedIdMap[id] === "undefined") {
+				if(!usedIdMap[id]) {
 					usedIds.push(id);
-					usedIdMap[id] = id;
+					usedIdMap[id] = true;
 				}
 			});
 		}
 
+		// TODO
 		var modules1 = this.modules;
 		for(var indexModule1 = 0; indexModule1 < modules1.length; indexModule1++) {
 			var module1 = modules1[indexModule1];
-			if(module1.id !== null && typeof usedIdMap[module1.id] === "undefined") {
+			if(module1.id && !usedIdMap[module1.id]) {
 				usedIds.push(module1.id);
-				usedIdMap[module1.id] = module1.id;
+				usedIdMap[module1.id] = true;
 			}
 		}
 
 		if(usedIds.length > 0) {
-			const usedNumberIds = usedIds.filter(id => typeof id === "number");
-			nextFreeModuleId = usedNumberIds.reduce((a, b) => Math.max(a, b), -1) + 1;
-			for(let i = 0; i < nextFreeModuleId; i++) {
-				if(usedIdMap[i] !== i)
-					unusedIds.push(i);
+			var usedIdMax = -1;
+			for(var index = 0; index < usedIds.length; index++) {
+				var usedIdKey = usedIds[index];
+
+				if(typeof usedIdKey !== "number") {
+					continue;
+				}
+
+				usedIdMax = Math.max(usedIdMax, usedIdKey);
+			}
+
+			var lengthFreeModules = nextFreeModuleId = usedIdMax + 1;
+
+			while(lengthFreeModules--) {
+				if(!usedIdMap[lengthFreeModules]) {
+					unusedIds.push(lengthFreeModules);
+				}
 			}
-			unusedIds.reverse();
 		}
 
 		var modules2 = this.modules;
@@ -969,11 +982,12 @@ class Compilation extends Tapable {
 
 		if(this.usedChunkIds) {
 			nextFreeChunkId = getNextFreeChunkId(this.usedChunkIds) + 1;
-			for(let i = 0; i < nextFreeChunkId; i++) {
-				if(this.usedChunkIds[i] !== i)
-					unusedIds.push(i);
+			var index = nextFreeChunkId;
+			while(index--) {
+				if(this.usedChunkIds[index] !== index) {
+					unusedIds.push(index);
+				}
 			}
-			unusedIds.reverse();
 		}
 
 		var chunks = this.chunks;
@@ -1165,8 +1179,9 @@ class Compilation extends Tapable {
 			try {
 				const useChunkHash = !chunk.hasRuntime() || (this.mainTemplate.useChunkHash && this.mainTemplate.useChunkHash(chunk));
 				const usedHash = useChunkHash ? chunkHash : this.fullHash;
-				if(this.cache && this.cache[`c${chunk.id}`] && this.cache[`c${chunk.id}`].hash === usedHash) {
-					source = this.cache[`c${chunk.id}`].source;
+				const cacheName = "c" + chunk.id;
+				if(this.cache && this.cache[cacheName] && this.cache[cacheName].hash === usedHash) {
+					source = this.cache[cacheName].source;
 				} else {
 					if(chunk.hasRuntime()) {
 						source = this.mainTemplate.render(this.hash, chunk, this.moduleTemplate, this.dependencyTemplates);
@@ -1174,7 +1189,7 @@ class Compilation extends Tapable {
 						source = this.chunkTemplate.render(chunk, this.moduleTemplate, this.dependencyTemplates);
 					}
 					if(this.cache) {
-						this.cache[`c${chunk.id}`] = {
+						this.cache[cacheName] = {
 							hash: usedHash,
 							source: source = (source instanceof CachedSource ? source : new CachedSource(source))
 						};
diff --git a/lib/optimize/RemoveParentModulesPlugin.js b/lib/optimize/RemoveParentModulesPlugin.js
index 6612848c0..880ebc35d 100644
--- a/lib/optimize/RemoveParentModulesPlugin.js
+++ b/lib/optimize/RemoveParentModulesPlugin.js
@@ -24,35 +24,35 @@ function hasModule(chunk, module, checkedChunks) {
 
 function allHaveModule(someChunks, module, checkedChunks) {
 	if(!checkedChunks) checkedChunks = [];
-	let chunks = [];
+	var chunks = [];
 	for(var i = 0; i < someChunks.length; i++) {
 		checkedChunks.push(someChunks[i]);
-		const subChunks = hasModule(someChunks[i], module, checkedChunks);
+		var subChunks = hasModule(someChunks[i], module, checkedChunks);
 		if(!subChunks) return false;
-		addToSet(chunks, subChunks);
+
+		for(var index = 0; index < subChunks.length; index++) {
+			var item = subChunks[index];
+
+			if(!chunks.length || chunks.indexOf(item) < 0) {
+				chunks.push(item);
+			}
+		}
 	}
 	return chunks;
 }
 
-function addToSet(set, items) {
-	for(var index = 0; index < items.length; index++) {
-		var item = items[index];
-
-		if(set.indexOf(item) < 0) {
-			set.push(item);
-		}
-	}
-}
-
 function debugIds(chunks) {
-	const list = chunks.map((chunk) => {
-		return chunk.debugId;
-	});
-	const debugIdMissing = list.some((dId) => {
-		return typeof dId !== "number";
-	});
-	if(debugIdMissing)
-		return "no";
+	var list = [];
+	for(var i = 0; i < chunks.length; i++) {
+		var debugId = chunks[i].debugId;
+
+		if(typeof debugId !== "number") {
+			return "no";
+		}
+
+		list.push(debugId);
+	}
+
 	list.sort();
 	return list.join(",");
 }
@@ -70,9 +70,9 @@ class RemoveParentModulesPlugin {
 						var module = modules[i];
 
 						if(chunk.parents.length === 0) continue;
-						var dId = "$" + debugIds(module.chunks);
+						var dId = debugIds(module.chunks);
 						var parentChunksWithModule;
-						if((dId in cache) && dId !== "$no") {
+						if((dId in cache) && dId !== "no") {
 							parentChunksWithModule = cache[dId];
 						} else {
 							parentChunksWithModule = cache[dId] = allHaveModule(chunk.parents, module);