How to trace what causes slow loading times?

So far I reduced the test page to the simplest html page:

<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>

<h1>Test Empty Page</h1>

</body>
</html>

But still when I open devtools, and go to Network tab, I see that localhost loading time is 1.5 seconds.

How should I start tracing that? Is it something in Providers? Kernel? maybe too much junk in public folder? Although I am not loading anything from there in this empty page (They didn't use mix, just copied resources directly to public). Or - something else where they put maybe code that should not be there and makes the page load slowly?

Update: Most of the time it loads in short times, for example 200 ms, however there are many times where it just spikes to 1.5 seconds. So maybe they connected the app to some external source..? Or this is the DB connection? Not sure

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