Add performance tips to timeline docs (#3360)

docs/timeline/index.html

@@ -2113,6 +2113,15 @@ var options = {
 </style>
 </pre>
 
+ <h3 id="Performance_Tips">Performance  Tips</h3>
+ <p>Defining a timeline with many items and/or groups might affect initial loading time and general performance.
+   Here are some tips to improve performance and avoid slow loading time:</p>
+ <ul>
+   <li>Define items and group with <code>DataSets</code></li>
+   <li>Avoid applying heavy CSS on items (such as <code>box-shadow</code>, gradient background colors, etc.)</li>
+   <li>Defining a <code>start</code> and an <code>end</code> in the timeline options.
+   This will improve initial loading time.</li>
+ </ul>
 </div>
 
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