How can the speed of interaction between the client and the server when loading a webpage be affected?

What can affect the speed of interaction when loading a webpage between a client and a server ?

How can it be affected depending on what the client and server computer are doing, how they are connected, and what they consist of ?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).