Very slow loading site

The site was created using version 3.0.2.0.
At first it worked fine, now I notice that the site is loading very slowly. Really very slow.
Website address: http://www.trikotazaslt.lt/

How could this problem be addressed?

This is hard to say without further details. It could be your web theme which is a bit confusing and probably not OpenCart compliant, try clearing your vqmod caches, it there. And try the default theme to see whether it makes a difference.

Probably your theme.

Switch back to default to test.

If it is indeed the theme, insist the theme developer fixes it or refunds you.

Your homepage (link you provided) looks clean enough, no html errors, but you have a very large quantity of Stylesheets, Javascripts, fonts, and images being called in your <header>, most of them not minified:

<script src="catalog/view/javascript/jquery/jquery-2.1.1.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/bootstrap/js/bootstrap.min.js" type="text/javascript"></script>
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5 ... ss/all.css" integrity="sha384-DNOHZ68U8hZfKXOrtjWvjxusGo9WQnrNx2sqG0tfsghAvtVlRW3tvkXWZh58N9jp" crossorigin="anonymous">
<link href="catalog/view/javascript/jquery/swiper/css/swiper.min.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/javascript/jquery/swiper/css/opencart.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/javascript/owl-carousel/css/owl.carousel.min.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/javascript/owl-carousel/css/owl.theme.default.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/pav_strollik/stylesheet/pavobuilder.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/default/stylesheet/pavobuilder/5a274b878412c.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/pav_strollik/stylesheet/sliderlayer/css/typo.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/default/stylesheet/pavobuilder/5a2790dae25e1.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/default/stylesheet/d_ajax_search/d_ajax_search.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/javascript/jquery/magnific/magnific-popup.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/javascript/jquery/pavquickview.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/javascript/font-awesome/css/font-awesome.min.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/pav_strollik/stylesheet/stylesheet.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/pav_strollik/stylesheet/skins/1522852352.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/pav_strollik/stylesheet/customize.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/default/stylesheet/pavobuilder/5a338ae076234.css" type="text/css" rel="stylesheet" media="screen" />
<link href="catalog/view/theme/default/stylesheet/pavobuilder/5a3cccd2abd07.css" type="text/css" rel="stylesheet" media="screen" />
<script src="catalog/view/javascript/pavobuilder/dist/pavobuilder.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/jquery/swiper/js/swiper.jquery.js" type="text/javascript"></script>
<script src="catalog/view/javascript/owl-carousel/owl.carousel.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/layerslider/jquery.themepunch.revolution.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/d_tinysort/tinysort.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/d_tinysort/jquery.tinysort.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/jquery/magnific/jquery.magnific-popup.min.js" type="text/javascript"></script>
<script src="catalog/view/theme/pav_strollik/javascript/pavquickview.min.js" type="text/javascript"></script>
<script src="catalog/view/javascript/jquery/swiper/js/swiper.js" type="text/javascript"></script>
<script src="catalog/view/theme/pav_strollik/javascript/common.js" type="text/javascript"></script>
<link href="http://www.trikotazaslt.lt/image/catalog/krepselist.png" rel="icon" />

Surely that is too many.

My advice is:

First, try to minify as many scripts and css as you can.
Second, move all secondary scripts (like sliders) in your footer instead.
Third, run test on services like "PingDom", "WebPageTest" or "GTMetrics"

Good luck!

I don't think it could be a template problem because there was no such problem on the site before, although the template was used the same.
Secondly, the admin page also works. What could be other problems?

So have you tried it with the default theme, just to be sure?