Grammar for Everyone - Barbara Dykes 2007

Comparative adverbs
Introduction
Part II The parts of speech

Adverbs of comparison follow a similar pattern to comparative adjectives while maintaining their function of modifying words.

For example:

fast, faster [comparing two], fastest [comparing more than two]

The cake disappeared fast.

Your buns went faster.

But the pizza went fastest!

In the case of longer adverbs we use more and most - again to avoid clumsiness.

What a colourful tie. This one’s more colourful.

But that one is most colourful.