A specific IP is making a large number of unwanted requests to your site, and which is impacting the site and server's performance.
You can quickly block that IP from accessing your site, via the site's configuration. The steps to do this are slightly different on Drupal 7 and Drupal 8 sites due to differences in core, with documentation on www.drupal.org providing a guide on both blocking IP's on Drupal 7 and on blocking IP's on a Drupal 8 site. Note though that this approach is only practicable for a small number of IP's, and is not scalable against a widespread DDoS attack.
As a further step we recommend implementing an IP block in the .htaccess on the server, as this prevents unwanted IP's from even reaching your Drupal site.
As a longer tern solution, the best approach in managing requests from unwanted sources, is via a service like Acquia Cloud Edge. Acquia Cloud Edge is powered by Cloudflare and delivers advanced (D)DoS protection and CDN services. This is also a much more scalable solution than manually blocking IP's or user agents during or after an event. Please contact your account manager if you're interested in finding out more about the features available with Acquia Cloud Edge.