When anticipating a large traffic burst on your site, it can be difficult to measure how your site will perform when under pressure. There are a number of factors which can determine your success through the surge of traffic, but the main area to focus on is the overall cacheability of your site.
The more cacheable your site is, the easier it is for the server to serve each page request. This means the site can scale to support more traffic at once.
WP Engine’s page caching layer is purged every 10 minutes. This means the server doesn’t have to build each page as “new” each time a visitor requests it.
Caching offloads a lot of the work the server would normally have to go through to generate each request on your site, meaning it can handle more page requests simultaneously.
One thing you can do to help combat and offset the number of uncached pages on your site is to extend the default cache expiration time. You can use the WP Engine Advanced Cache plugin to configure cache settings for optimal cacheability.