Should improve performance of closest considerably. Benchmark proof in speed/closest.html

speed/benchmark.js

 // Runs a function many times without the function call overhead
-function benchmark(fn, times){
+function benchmark(fn, times, name){
 	fn = fn.toString();
 	var s = fn.indexOf('{')+1,
 		e = fn.lastIndexOf('}');
 	fn = fn.substring(s,e);
 	
-	return new Function('i','var t=new Date;while(i--){'+fn+'};return new Date-t')(times);
+  return benchmarkString(fn, times, name);
+}
+
+function benchmarkString(fn, times, name) {
+  var fn = new Function("i", "var t=new Date; while(i--) {" + fn + "}; return new Date - t")(times)
+  fn.displayName = name || "benchmarked";
+  return fn;
 }

speed/closest.html

+<!doctype html>
+<html>
+<head>
+	<title>Test Event Handling Performance</title>
+	<script src="benchmark.js"></script>
+	<script src="jquery-basis.js"></script>
+	<script>var old = jQuery.noConflict(true);</script>
+	<script src="../dist/jquery.js"></script>
+  <script>
+    jQuery(function ready() {
+      var node = $("#child"), name;
+
+      [".zoo", "#zoo", "[data-foo=zoo]", "#nonexistant"].forEach(function foreach(item) {
+        name = "closest '" + item + "'";
+        console.log(name);
+
+        console.log("new", benchmarkString("$('#child').closest('" + item + "')", 5000, name));
+        console.log("old", benchmarkString("old('#child').closest('" + item + "')", 5000, name));
+      });
+    });
+	</script>
+</head>
+<body>
+  <div>
+    <p>Hello</p>
+    <div class="zoo" id="zoo" data-foo="bar">
+      <div>
+        <p id="child">lorem ipsum</p>
+        <p>dolor sit amet</p>
+      </div>
+    </div>
+  </div>
+</body>
+</html>
+

src/traversing.js

@@ -8,6 +8,8 @@ var runtil = /Until$/,
 	isSimple = /^.[^:#\[\.,]*$/,
 	slice = Array.prototype.slice;
 
+var POS = jQuery.expr.match.POS;
+
 jQuery.fn.extend({
 	find: function( selector ) {
 		// Handle "> div" child selectors and pass them to .children()
@@ -95,21 +97,27 @@ jQuery.fn.extend({
 			return ret;
 		}
 
-		var pos = jQuery.expr.match.POS.test( selectors ) ? 
+		var pos = POS.test( selectors ) ? 
 			jQuery( selectors, context || this.context ) : null;
 
-		ret = jQuery.map(this.get(),function( cur,i ) {
-			while ( cur && cur.ownerDocument && cur !== context ) {
-				if ( pos ? pos.index(cur) > -1 : jQuery(cur).is(selectors) ) {
-					return cur;
-				}
+    var ret = [];
 
-				cur = cur.parentNode;
-			}
+    for ( var i=0,j=this.length; i<j; i++ ) {
+      var cur = this[i];
+
+      while ( cur ) {
+        if ( pos ? pos.index(cur) > -1 : jQuery.find.matches(selectors, [cur]).length ) {
+					ret.push( cur );
+          break;
+				} else {
+          cur = cur.parentNode;
+          if ( !cur.ownerDocument || cur === context ) {
+            break;
+          }
+        }
+      }
+    }
 
-			return null;
-		});
-		
 		ret = ret.length > 1 ? jQuery.unique(ret) : ret;
 		
 		return this.pushStack( ret, "closest", selectors );