Update portal-performance.md

Added in reference to Page Diag tool and SPO Performance tuning

Author: Stealth7777

docs/solution-guidance/portal-performance.md

@@ -13,6 +13,16 @@ Implementing customizations to SharePoint Online places an even greater emphasis
 > [!NOTE] 
 > Although this guidance primarily targets SharePoint Online, most of it also applies to portals hosted in an on-premises SharePoint environment.
 
+## Optimizing Classic Portal Pages
+If you have not implemented Modern pages yet and are looking to optimize your existing or new Classic portal pages then this section applies to you.
+To assist with some intial page reviews and start the process of understanding performance on classic portal pages for SharePoint Online, the [Page Diagnostics tool for SharePoint](https://aka.ms/perftool) can be utilized.
+It is a Chrome extension developed by Microsoft to highlight guidance for optimzing Classic SharePoint portal pages. 
+
+Whilst some of the items highlighted relate to existing out of the box functionality, we are working towards removing these components as there are better alternatives that provide a faster user experience.The biggest culprit is the use of structural navigation.
+The tool also highlights enhanced functionlity e.g. Content Delivery Networks (CDNs), that have been made available by Microsoft to further optimize the end user experience. Please also see [Tune SharePoint Online Performance](https://aka.ms/spoperformance)
+
+What you will see is that between the Page Diagnostics tool and tuning guidance, they provide a high level overview of what impacts performance whilst the details on this page take you deeper into how customizations should be built to avoid impacting a pages performance. 
+
 <a name="bk_antiPatterns"> </a>
 
 ## What not to do
@@ -426,4 +436,4 @@ The logical architecture of the Data Access Layer (DAL) Framework includes the f
 - [Introduction to performance tuning for SharePoint Online](https://support.office.com/en-US/article/Introduction-to-performance-tuning-for-SharePoint-Online-81c4be5f-327e-435d-a568-526d68cffef0)
 - [Tune SharePoint Online performance](https://support.office.com/en-us/article/Tune-SharePoint-Online-performance-f0522d4a-fbf4-41f9-854e-c9b59555091d)
 - [Learn how to build a fast, responsive SharePoint portal in SharePoint Online](https://channel9.msdn.com/Events/Ignite/2016/BRK3026) (Channel 9 video)
-- [Building SharePoint Online portals](portal-overview.md)
+- [Building SharePoint Online portals](portal-overview.md)