Reason01What site did you first visit when you woke up in the morning?

Many people have probably visited the so-called GAFA sites, such as Google, Amazon, and Facebook. GAFA is the fastest in the world. Users who regularly use these sites have become accustomed to their speed.

Reason02User expectations are
high from the very beginning

Users who are always using the fastest sites in the world will feel that any site that is even slightly slower is difficult to use.
In other words, it is like users who are constantly watching the outstanding performances of Olympic athletes witnessing the amateur-level, unrefined performances.

Reason03 A slow site
leaves a worse impression than a site that crashes

For example, when a major site experiences a server outage due to high traffic, it may cause a temporary stir in the media, but users will return after the recovery. This is because they understand that server outages are temporary.
On the other hand, once users feel that a site is slow, they tend to avoid it. This is because slow sites are judged to be permanently difficult to use.

Source: Time is Money

Reason04Google does not deal with
slow sites

“If something is free, you are the product.”
To put this in simpler terms, it means that with free services, the users themselves become the product. Think from Google's perspective. Would you want to promote slow websites (that is, websites with poor user experience) at the top of search results?
As a result, slow websites see their SEO rankings drop, and users continue to drift away.