It'll probably be slower than smartly written, natively compiled code, but not necessarily by enough to matter. It really depends on what you're trying to do, so the best way to know is to try it and measure.
Compiling to a human-readable format can be pretty straightforward. You can skip some steps entirely (e.g. register selection), and modern tools make it easy to ignore others (lexing & parsing) for quite a while. Don't think about it as "compiling", if you think that's a scary hard thing. Think of it as "reading in a couple kinds of simple structures and converting them to text". Like a templating engine.
Figure out what kind of operations you want to support, how to map them into small chunks of Javascript, and go for it. You can pass hand-written JSON, XML, sexps, dicts, etc. to the compiler in a REPL and worry about parsing and other stuff later.
Compiling to a human-readable format can be pretty straightforward. You can skip some steps entirely (e.g. register selection), and modern tools make it easy to ignore others (lexing & parsing) for quite a while. Don't think about it as "compiling", if you think that's a scary hard thing. Think of it as "reading in a couple kinds of simple structures and converting them to text". Like a templating engine.
Figure out what kind of operations you want to support, how to map them into small chunks of Javascript, and go for it. You can pass hand-written JSON, XML, sexps, dicts, etc. to the compiler in a REPL and worry about parsing and other stuff later.