MySql or Xpath for high speed query results

[berege]

I have a database which stores a huge tree[in closure form]. If I use mysql to query, it takes a lot of time. I tried indexing but it does not give the needed speed.My website need is to create a module which will be used most of the time and cannot have delays in accessing the data[the tree]. This data is not prone to changes.So I am trying to save the entire data to a xml or json.Now,[*]If I am using xml, is XPath a better way to apply filters[or query] on my tree stored in xml?[*]If json is a better option, than how do I query json data?[*]Or, mySql is my best available option.