In the article it was examined what some of the tests for page load speed measure as TTFB: The tests measure the time until the arrival of the first character of the HTTP headers.

When using a dynamically generated page it is possible to send the HTTP headers, then maybe send some some content and then do some server side calculations or database access, and finally send the rest of the page content. The TTFB measured by the tests, which were mentioned in the article, will not reflect the time needed for server side calculations.

When using a server side scripting language there may be some kind of output buffering, which has to be deactivated first.