add comment about perf concern in setupBindListeners

src/bind/bind.html

@@ -25,6 +25,11 @@
           // <node>.on.<property>-changed: <path]> = e.detail.value
           //console.log('[_setupBindListener]: [%s][%s] listening for [%s][%s-changed]', this.localName, info.path, info.id || info.index, info.property);
 
+          // TODO(sorvell): the function generation here appears to be somewhat costly,
+          // showing up as ~5-10% of overall creation cost; we can fairly easily
+          // avoid this being a per instance cost. Note also that the calls to 
+          // addEventListener contribute another ~5-10%
+          //
           // TODO(kschaaf): using the dirty check from notifyPath to determine whether
           // to update the value avoids null references when the object has gone away
           var notifyPath = (info.path.indexOf('.') > 0) ?