The above REPL is running purely locally in the web browser. It is a faithful emulation of the behavior of the Gambit Scheme interpreter run from the shell (including editing the command line, tab completion, ctrl-C, local filesystem, green threads, etc).

Try one of these demos: 1000 digits of π simple debugging getting help list files view file jseval Gambit logo

The performance will depend on the browser and version you are using. Here is a table summarizing the performance of the fib benchmark on a 2012 notebook on 3 browsers and when compiled natively with gcc:

gcc56 ms
Firefox 28.0a1982 ms
Chrome Version 30.0.1599.1011135 ms
Safari Version 7.02782 ms

This site was built using the following open-source softwares: Emscripten, Terminus, and Gambit Scheme.