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And use 80% of the resources?

Then 80% aren't, and those ones will perform very well.

The other 20% also perform well. I have quite literally never encountered a Google search that took more than some tens of milliseconds for a round trip.

You're absolutely correct. Caching common search queries allows the site to allocate hardware for processing "expensive" queries, with the objective of having both complete at near the same time.

Without caching, the cost of operating the site would dramatically escalate.

Eh, a non trivial % of those are likely just brand new misspellings.