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).
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:
|Chrome Version 30.0.1599.101
|Safari Version 7.0