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>

doc/api/vm.md

@@ -125,6 +125,10 @@ console.log(util.inspect(sandbox));
 // { animal: 'cat', count: 12, name: 'kitty' }
 ```
 
+*Note*: Using the `timeout` or `breakOnSigint` options will result in new
+event loops and corresponding threads being started, which have a non-zero
+performance overhead.
+
 ### script.runInNewContext([sandbox][, options])
 <!-- YAML
 added: v0.3.1