The easiest solution may be pdoTools which allows you to establish caching at run time.
Also, I do not believe resource placeholders are cached which is the best place to have your items cahced:
case '+': $tagName= substr($tagName, 1 + $tokenOffset); $element= new modPlaceholderTag($this->modx); $element->set('name', $tagName); $element->setTag($outerTag); $elementOutput= $element->process($tagPropString); break;
You may notice the other tag types have:
I cover the parser in Appendix D of my book. I found some issues in it in 2011 which Jason corrected.