Speed Issue with MU installation

I am working on a news portal (https://noticieropr.com) running on WordPress Multisite installation.

This site is taking too much time to load. After checking I found that the first-byte is returning after approx 30 seconds. And, some time it is running very fast.

I checked all settings with hosting provider. Even, migrated to new VPS server with same hosting provider. But, the issue is still same.

This site is the main site on Multisite setup. And, it is running on MyHosting.com VPS.

I also removed all unwanted plugins and unwanted tables created by inactive or removed plugins.

I am unable to detect any issue with WordPress setup.

Any one can help?