Learn Swedish: Den nya surfplattan är mycket snabbare.

Because the adjective is in the comparative form (snabbare, meaning 'faster'). Just like in English where we say 'much faster' instead of 'very faster', Swedish uses mycket (much) to intensify a comparative adjective. You would only use väldigt with the base form, like väldigt snabb (very fast).

Does the comparative adjective snabbare need to change because surfplattan is an en-word?

Nope! A great thing about Swedish comparative adjectives ending in -are is that they never change their endings. Whether the noun is an en-word, an ett-word, or plural, the comparative form stays exactly the same: snabbare.

I always forget the 'double definite' rule. Why do we need both Den and the -n on surfplattan?

When you put an adjective (nya) in front of a specific, definite noun, Swedish requires 'double definiteness'. You need the standalone definite article (den for en-words) before the adjective, AND the noun must still keep its definite ending at the end of the word (surfplattan). Don't forget that the adjective also gets an -a at the end (nya).

What kind of word is surfplatta?