How is it so? If you're referring to the Packrat memoisation (which is not the only possible PEG implementation), you can do a lot of memory optimisation, like discarding memoised entries based on some rules (e.g., once a top-level entry, like a function or a class definition is parsed, all the alternative interpretations can be thrown away). You can memoise complex entries but re-parse simple tokens. And many, many more.