diff --git a/lib/hd/mnemonic.js b/lib/hd/mnemonic.js
index af2a623e..fa3cd632 100644
--- a/lib/hd/mnemonic.js
+++ b/lib/hd/mnemonic.js
@@ -19,6 +19,12 @@ const wordlist = require('./wordlist');
 const common = require('./common');
 const nfkd = require('../utils/nfkd');
 
+/*
+ * Constants
+ */
+
+const wordlistCache = new Map();
+
 /**
  * HD Mnemonic
  * @alias module:hd.Mnemonic
@@ -191,7 +197,7 @@ Mnemonic.prototype.getPhrase = function getPhrase() {
 
   // Build the mnemonic by reading
   // 11 bit indexes from the entropy.
-  const wordlist = Mnemonic.getWordlist(this.language);
+  const list = Mnemonic.getWordlist(this.language);
 
   let phrase = [];
   for (let i = 0; i < bits / 11; i++) {
@@ -203,7 +209,7 @@ Mnemonic.prototype.getPhrase = function getPhrase() {
       index <<= 1;
       index |= (entropy[oct] >>> (7 - bit)) & 1;
     }
-    phrase.push(wordlist[index]);
+    phrase.push(list.words[index]);
   }
 
   // Japanese likes double-width spaces.
@@ -224,6 +230,9 @@ Mnemonic.prototype.getPhrase = function getPhrase() {
  */
 
 Mnemonic.prototype.fromPhrase = function fromPhrase(phrase) {
+  assert(typeof phrase === 'string');
+  assert(phrase.length <= 1000);
+
   const words = phrase.split(/[ \u3000]+/);
   let bits = words.length * 11;
   const cbits = bits % 32;
@@ -240,14 +249,14 @@ Mnemonic.prototype.fromPhrase = function fromPhrase(phrase) {
   ent.fill(0);
 
   const lang = Mnemonic.getLanguage(words[0]);
-  const wordlist = Mnemonic.getWordlist(lang);
+  const list = Mnemonic.getWordlist(lang);
 
   // Rebuild entropy bytes.
   for (let i = 0; i < words.length; i++) {
     const word = words[i];
-    const index = wordlist.indexOf(word);
+    const index = list.map.get(word);
 
-    if (index === -1)
+    if (index == null)
       throw new Error('Could not find word.');
 
     for (let j = 0; j < 11; j++) {
@@ -267,9 +276,9 @@ Mnemonic.prototype.fromPhrase = function fromPhrase(phrase) {
   for (let i = 0; i < cbits; i++) {
     const bit = i % 8;
     const oct = (i - bit) / 8;
-    const a = (chk1[oct] >>> (7 - bit)) & 1;
-    const b = (chk2[oct] >>> (7 - bit)) & 1;
-    if (a !== b)
+    const b1 = (chk1[oct] >>> (7 - bit)) & 1;
+    const b2 = (chk2[oct] >>> (7 - bit)) & 1;
+    if (b1 !== b2)
       throw new Error('Invalid checksum.');
   }
 
@@ -337,8 +346,8 @@ Mnemonic.fromEntropy = function fromEntropy(entropy, lang) {
 
 Mnemonic.getLanguage = function getLanguage(word) {
   for (const lang of Mnemonic.languages) {
-    const wordlist = Mnemonic.getWordlist(lang);
-    if (wordlist.indexOf(word) !== -1)
+    const list = Mnemonic.getWordlist(lang);
+    if (list.map.has(word))
       return lang;
   }
 
@@ -347,12 +356,22 @@ Mnemonic.getLanguage = function getLanguage(word) {
 
 /**
  * Retrieve the wordlist for a language.
- * @param {String} language
- * @returns {String[]}
+ * @param {String} lang
+ * @returns {Object}
  */
 
-Mnemonic.getWordlist = function getWordlist(language) {
-  return wordlist.get(language);
+Mnemonic.getWordlist = function getWordlist(lang) {
+  const cache = wordlistCache.get(lang);
+
+  if (cache)
+    return cache;
+
+  const words = wordlist.get(lang);
+  const list = new WordList(words);
+
+  wordlistCache.set(lang, list);
+
+  return list;
 };
 
 /**
@@ -530,6 +549,23 @@ Mnemonic.isMnemonic = function isMnemonic(obj) {
     && typeof obj.toSeed === 'function';
 };
 
+/**
+ * Word List
+ * @constructor
+ * @ignore
+ * @param {Array} words
+ */
+
+function WordList(words) {
+  this.words = words;
+  this.map = new Map();
+
+  for (let i = 0; i < words.length; i++) {
+    const word = words[i];
+    this.map.set(word, i);
+  }
+}
+
 /*
  * Expose
  */