A neat little collection of tricks to make an interpreted, dynamically typed language fast.
The short version is: a compact value representation, fixed object layout, copy-down inheritance, method signatures, computed gotos and a single-pass compiler.
The entire thing seems to be inspired by Lua, which is also known to be quite fast.
The author of Wren, Bob Nystrom, also wrote a book about implementing languages, which looks to be really neat as well. The existing parts are already excellent, but for the unfinished low-level details you’ll have to look elsewhere for now.