There are the following reasons which make slow the websites:
1. Compressible Resources
All modern browsers support and automatically negotiate gzip compression for all HTTP requests. Enabling gzip compression can reduce the size of the transferred response by up to 90%, which can significantly reduce the amount of time to download the resource, reduce data usage for the client, and improve the time to first render of your pages.
2. Server Response Time
Server response time measures how long it takes to load the necessary HTML to begin rendering the page from your server, subtracting out the network latency between Google and your server. You should reduce your server response time under 200ms.
3. Inline CSS
Modern browsers block on external CSS before painting content to the screen. This incurs additional network latency and increases the time it takes to display content to the screen. To optimize the time to render, if the external CSS resources are small, you can insert those directly into the HTML document. Inlining small CSS in this way allows the browser to proceed with rendering the page.
4. Leverage Browser Caching
Fetching resources over the network is both slow and expensive: the download may require multiple roundtrips between the client and server, which delays processing and may block rendering of page content, and also incurs data costs for the visitor. All server responses should specify a caching policy to help the client determine if and when it can reuse a previously fetched response.
5. Unnecessary or redundant data
Minification refers to the process of removing unnecessary or redundant data without affecting how the resource is processed by the browser – e.g. code comments and formatting, removing unused code, using shorter variable and function names, and so on.
6. Unoptimized Images
Images often account for most of the downloaded bytes on a page. As a result, optimizing images can often yield some of the largest byte savings and performance improvements: the fewer bytes the browser has to download, the less competition there is for the client’s bandwidth and the faster the browser can download and render content on the screen.