I will assume that the website you tested is dynamic. It is probably created in PHP, ASP or another scripting language producing an output based on the operation of the code being executed (other than HTML) and on the inputs that are passed on it (including the # 39; IP address of the user, etc.).
When the script and server resources are misused, the result is long waiting times.
For example, suppose the website has a huge database in the background, with about a billion records. Seeing that I work with databases myself, trying to count on a huge amount of records, even on a fast system, takes time in milliseconds, and these milliseconds add up, and if the script that produces the page you are trying to display ends If you try to count the billion records repeatedly throughout the script without caching the results, the wait time can be very long and I would personally want to fire the programmer from the company if I had the power.