doc: document vm timeout option perf impact

Mention that the `timeout` option has a noticeable performance impact.

Fixes: https://github.com/nodejs/node/issues/10453
PR-URL: https://github.com/nodejs/node/pull/12751


Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>