The structure you gave is A valid structure, but it is not the ONLY valid structure.
Grammatically, you can use a superlative in any or almost any sentence structure in which you could use an adjective. Of course the specific words have to make sense in context, but a superlative is an adjective and can be used like one.
There's the structure you gave: "Our Internet is fast." "Our Internet is the fastest."
But adjectives are used in many other ways. "Today I drove a red car." "Today I drove the fastest car."
"Bob, a bald man, took off his hat." "Bob, the biggest man, took off his hat."