diff --git a/lib/bcoin/block.js b/lib/bcoin/block.js
index b7287e99..2a488bb3 100644
--- a/lib/bcoin/block.js
+++ b/lib/bcoin/block.js
@@ -456,7 +456,7 @@ Block.prototype.getClaimed = function getClaimed() {
  * @returns {BN}
  */
 
-Block.reward = function reward(height, network) {
+Block.reward = function _reward(height, network) {
   var halvings, reward;
 
   network = bcoin.network.get(network);
@@ -468,11 +468,25 @@ Block.reward = function reward(height, network) {
   if (halvings >= 64)
     return 0;
 
+  // We need to shift right by `halvings`,
+  // but 50 btc is a 33 bit number, so we
+  // cheat. We only start halving once the
+  // halvings are at least 1.
   if (halvings === 0)
     return 5000000000;
 
+  // We can't shift right by 32 bits.
+  // 25m bitcoin is 32 bits, so we
+  // can safely return zero if the
+  // shift will be 32.
+  if (halvings >= 33)
+    return 0;
+
   reward = 2500000000 >>> (halvings - 1);
 
+  // We could do this to keep this more (or
+  // less?) accurate, but this is infinitely
+  // slower.
   // reward = 5000000000;
   // reward = Math.floor(reward / Math.pow(2, halvings));
 
diff --git a/lib/bcoin/protocol/constants.js b/lib/bcoin/protocol/constants.js
index f50c28d2..426ab9fb 100644
--- a/lib/bcoin/protocol/constants.js
+++ b/lib/bcoin/protocol/constants.js
@@ -9,7 +9,6 @@
  * @module constants
  */
 
-var bn = require('bn.js');
 var utils = require('../utils');
 
 /**
@@ -413,7 +412,7 @@ exports.tx = {
   MAX_FEE: exports.COIN / 10,
   MIN_RELAY: 10000,
   BARE_MULTISIG: true,
-  FREE_THRESHOLD: Math.floor(exports.COIN * 144 / 250),
+  FREE_THRESHOLD: exports.COIN * 144 / 250,
   MAX_SIGOPS: exports.block.MAX_SIGOPS / 5,
   MAX_SIGOPS_COST: exports.block.MAX_SIGOPS_COST / 5,
   COINBASE_MATURITY: 100
diff --git a/lib/bcoin/tx.js b/lib/bcoin/tx.js
index c22dfda0..45115db6 100644
--- a/lib/bcoin/tx.js
+++ b/lib/bcoin/tx.js
@@ -80,6 +80,8 @@ function TX(data, block, index) {
   this._raw = data._raw || null;
   this._size = data._size || 0;
   this._witnessSize = data._witnessSize || 0;
+  this._outputValue = null;
+  this._inputValue = null;
 
   for (i = 0; i < data.inputs.length; i++)
     this.inputs.push(new bcoin.input(data.inputs[i]));
@@ -665,12 +667,18 @@ TX.prototype.getInputValue = function getInputValue() {
   var total = 0;
   var i;
 
+  if (this._inputValue != null)
+    return this._inputValue;
+
   if (!this.hasCoins())
     return total;
 
   for (i = 0; i < this.inputs.length; i++)
     total += this.inputs[i].coin.value;
 
+  if (!this.mutable)
+    this._inputValue = total;
+
   return total;
 };
 
@@ -683,9 +691,15 @@ TX.prototype.getOutputValue = function getOutputValue() {
   var total = 0;
   var i;
 
+  if (this._outputValue != null)
+    return this._outputValue;
+
   for (i = 0; i < this.outputs.length; i++)
     total += this.outputs[i].value;
 
+  if (!this.mutable)
+    this._outputValue = total;
+
   return total;
 };