Working on a document that is longer than 10 pages or so from within Smartview can be a resource-intensive activity for a browser and computer. It specifically depends on the number of entities (names, places, etc) it needs to cross-reference in the document.
For large documents with dense data older / less powerful computers may struggle to perform these actions in a browser - as such the page may become unresponsive. We'd advise waiting to see if the page eventually loads, but it may not be possible.
If this is the case, we'd recommend using bulk redaction instead. This performed entirely server-side within the backend of the system and doesn't rely on your computer's capabilities.