In general, Lua is usually faster than Ruby and JS because the language itself seems to have been engineered with a very clear understanding of where the language could be expressive/dynamic without unnecessarily sacrificing efficiency, and it's had a long time to mature. LPEG was written by one of the primary Lua authors, and it shows.

If you're geeked about PEGs, I highly recommend reading the paper and then the source. There are some details specific to the Lua C API, but it shouldn't be that hard to port or understand in isolation.