A couple of months ago I had the same issue. Make sure that default search is set to Elastic search (I used Sphinx which caused the issue).
If you are on a development server you might also consider disabling Amasty Improved layered Navigation and see if layered navigation works with search results.
I have checked the access log at /var/log/apache2/access.log
I found that SemrushBot, AhrefsBot, and PetalBot was making the connections.
I have added the below content to the custom instruction of robots.txt File at the
To troubleshoot CPU issues and load time, we can use several tools, such as top or pidstat.
To learn more about its installation and usage see this guide