Search engines can create a heavy load.
To solve the problem, add a Crawl-delay directive to robots.txt.
The Crawl-delay directive tells the search bot to load your site’s pages with an interval of at least 15 seconds, in other words, to pause while indexing the site’s pages.
Example
In your site’s root, create a robots.txt file and add:
User-agent: *
Crawl-delay: 15