Setting Read-Only Mode for an Entire Page

   To display a page in read-only mode you can use page attribute as follows:

<apex:page controller="myController" readOnly="true">
<apex:page>

Other advantage is query limit is getting increased from 50,000 rows to 1,000,000 rows.

In addition to querying many more rows, the readOnly attribute also increases the maximum number of items in a collection that can be iterated over using components such as <apex:dataTable>, <apex:dataList>, and <apex:repeat>. This limit increased from 1,000 items to 10,000. 
Special Note: 
https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_controller_readonly_context_methodlevel.htm