the page loading time on my machine is 55ms, but on test websites like pingdom.com, it's 1.5 seconds, why would that happen?

When I analyze the loading speed of website pages in Chrome, I find that the wait time (where the response files returned by the server are generated) is 55 ms and that the time of Total loading of the page is 1 second or less. When I run the test via https://www.pingdom.com/, http://gtmetrix.com/or the Google Speed ‚Äč‚ÄčTest Tool, the wait time is 1.8 seconds or more and the total load time of the page is 3 seconds or more.

The page is fully cached and the cache has been heated.

Any ideas on why this would happen? The efforts I have made for Google to see my site because the fast loading will be lost.

Do not comment on any other optimization tips like gzip and minification of css files. I am just wondering about the waiting time and / or its effects on ttfb.