Learn Spanish (Spain): Tanto la velocidad como esta característica son importantes. - Both the speed and this feature are important.

No. In the tanto [A] como [B] structure, tanto is an invariable adverb. It never changes to tanta, tantos, or tantas here, regardless of the gender or number of the nouns that follow it.

Why do we use tanto... como... here instead of a word like ambos?

While ambos means the pronoun or adjective form of both, it is used to group two things together into a single unit. The tanto... como... structure is used specifically to link two distinct elements in a sentence, matching the English formula for linking: both [A] and [B].

If both la velocidad and esta característica are singular, why are the verb and adjective plural (son importantes)?

The phrase tanto... como... creates a compound subject by adding two things together. Because you are talking about two distinct things (the speed and the feature), they form a plural subject together, which requires the plural verb son and the plural adjective importantes.

Do we have to include the article la and the demonstrative esta inside this structure?