Website note on JSON.stringify() fast path

website/guide/performance.html

@@ -456,3 +456,19 @@ function fast(x) {
 <p>Use such optimizations only where it matters, because they often reduce
 code readability.</p>
 
+<h2 id="performance-json-stringify-fastpath">JSON.stringify() fast path</h2>
+
+<p>There's a fast path for <code>JSON.stringify()</code> serialization which
+is used when there is no "replacer" argument and no indent.  The fast path
+assumes that it can serialize the argument value without risking any side
+effects (which might mutate the value being serialized), and will fall back
+to the slower default algorithm if necessary.  This happens at least when:
+(1) any object has a <code>.toJSON()</code> property, and (2) any object
+property is a getter.  See:
+<a href="https://github.com/svaarala/duktape/blob/master/tests/ecmascript/test-bi-json-enc-fastpath.js">test-bi-json-enc-fastpath.js</a>
+for detailed notes on current limitations; the fast path preconditions and
+limitations are very likely to change between Duktape releases.</p>
+
+<p>The fast path is currently not enabled by default.  To enable, ensure
+<code>DUK_USE_JSON_STRINGIFY_FASTPATH</code> is enabled in your
+<code>duk_config.h</code>.</p>