MultiSite running really slow and has large error log

Hello, your team was really extra helpful getting this multi site set up. It has been working well for a few months. Recently the site began running slowly and has a large error log. Could you help me determine what is causing this error. It is a cookie or http or error in line 99 error. I will paste the error below. It is kind of long, but is kind of repeating continuously.
[03-Jan-2020 18:09:18 UTC] PHP Warning: Use of undefined constant ‘HTTP_HOST’ – assumed ‘‘HTTP_HOST’’ (this will throw an Error in a future version of PHP) in /home/subtle11/public_html/wp-config.php on line 99