Pluck out the old wait instead of using splice.

 % node benchmark/increment/notify.js
 signal notify 1 x 5,274,081 ops/sec ±2.40% (92 runs sampled)
_signal notify 1 x 4,251,434 ops/sec ±3.44% (94 runs sampled)
 signal notify 2 x 5,296,992 ops/sec ±3.53% (88 runs sampled)
_signal notify 2 x 4,247,333 ops/sec ±0.80% (93 runs sampled)
 signal notify 3 x 5,393,405 ops/sec ±2.60% (90 runs sampled)
_signal notify 3 x 4,259,548 ops/sec ±0.83% (93 runs sampled)
 signal notify 4 x 5,447,035 ops/sec ±0.61% (95 runs sampled)
_signal notify 4 x 4,173,330 ops/sec ±2.69% (93 runs sampled)
Fastest is  signal notify 4, signal notify 2

_signal.js

@@ -60,7 +60,7 @@ Signal.prototype.notify = function () {
         // cancel it. We're going to want to be able cancel both exiting waits
         // and waits added during the notification.
         var waits = this._waits
-        this._cancels.unshift(this._waits = [])
+        this._cancels.push(this._waits = [])
 
         // We shift first so we don't wreck the array if a wait cancels itself.
         while (waits.length != 0) {

signal.js

@@ -71,7 +71,18 @@ Signal.prototype.notify = function () {
             waited.callback.apply(null, arguments)
         }
 
-        this._cancels.splice(this._cancels.indexOf(waits), 1)
+        var i = 0
+        while (i < this._cancels.length) {
+            if (this._cancels[i] === waits) {
+                break
+            }
+            i++
+        }
+        while (i <= this._cancels.length - 1) {
+            this._cancels[i] = this._cancels[i + 1]
+            i++
+        }
+        this._cancels.length--
     }
 }