Time To First Byte sample page.
This page is written in HTML. It loads instantaneously. No WordPress core, plugin delays, or theme overhead is associated with it.
At WebPagetest.org, you can test for the TTFB server delay by copying and pasting the browser displayed URL (above). WebPagetest.org.
The TTFB is labeled as First Byte. It appears in the Performance Results - first column from the left.
We consider Time To First Byte as your minimum server overhead. An excellent TTFB measurement is 200 milliseconds. Typical TTFB is around 400ms to 500ms. And bad is anything over 1 second. In that worst case, plugins are repeatedly hammering server resources. That slows down the server. These are usually the heaviest and popular plugins. They are writing and reading data and creating databases. Plugins like SEO, security, broken-link checkers, backup, caching (examples: SuperCache, W3-Total Cache), dynamic site map generators, related posts, etc.REFERENCE: common blacklisted plugins