Websites designed for mobile screens should be performant than usual

The constraint that designers have to work with in mobile is its screen size. Given that mobile screens are a lot smaller than desktop, the content that can be shown in a screen is less. Due to the lesser content, users will typically be moving a lot faster on mobile. They will swipe and scroll around very quickly to find what they want. The way a website is designed may turn out to have a deeper page hierarchy, translating to more clicks needed to get where the users needed to be. If a website is not performant enough to handle this behaviour, the users will get frustrated and leave.

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References

Krug, Don’t make me think (p. 148).

In some cases, the lack of space on each screen means that mobile sites become much deeper than their full-size cousins, so you might have to tap down three, four, or five "levels" to get some features or content. This means that people will be tapping more, but that's OK.