PIE calls a very expensive function -- getBoundingClientRect -- every time a property changes. MooTools adds dozens of properties to every element. In my case, this caused PIE to take 12 seconds to initialize in IE 8. Here's the fix in propChanged()

if( initialized && !( event && event.propertyName in ignorePropertyNames ) ) {
if (el[event.propertyName] && el[event.propertyName].apply) {
ignorePropertyNames[event.propertyName] = 1;
}
else {
update( 1 );
}