Why is website speed crucial for it’s success?

Who cares? It's just 4 seconds out of everyones time, it doesn't matter.

Well it does. Here is why

It is 4 seconds of human lifetime and 4 seconds of your visitors computer worktime (that kinda makes it 8 seconds now, but nevermind). That means it is 4 seconds of wasted electricity use in every device that had to visit 4 pages on your website.

Nobody can even notice such a loss of 4 or even 10 seconds in their day. True for humans, false for computers.

Every search engine that wants to have up-to-date information about websites, needs to crawl them.

That means, an automatic computer opens every page of your website and scans it for new information. You do not have to pay for this, as this is the basis of operation for search engines. This is what they need to do, in order to sell their ads.

So having a website that loads slower increases the worktime of search engines computers. Search engines do not want to burn money for no reason, so they optimize the process to balance their costs.

That is why all websites have a crawl budget: The amount of time and resources that Google devotes to crawling a site is commonly called the site's crawl budget. source: https://developers.google.com/search/docs/crawling-indexing/large-site-managing-crawl-budget#general_theory

Crawl budget, or not, this is just 1 second, right?

Well, no. This is 1 second per each page you have.

It is not uncommon for ecommerce websites to have 1000+ pages. That means, Google would waste 1000+ seconds (over 16 minutes) of machine time just to scan your website. And to be up to date, they want to scan it every single day.

Over a year, that is more than 100 hours of wasted machine worktime.

Don't be too worried though. These days we use asynchronous crawlers, that run on multiple top-end high thread count, high gigahertz machines. They have resources to crawl any website.

They have their own software (which is constantly improved) and they need to maintain their machines which are fixed costs, but the varying cost is electricity. To make as much profit as they can, they will naturally attempt to cut their bills.

This is why, good performance websites will always be more suitable in the eyes of search engine companies.