Adjective “Fast” vs. Adverb “Quickly”

What is the difference between fast and quickly? Is quickly an adverb?  An adjective?

Fast and quickly are often used interchangeably. Is there any difference between them? 

Fast is an adjective and refers to speed. Quickly is an adverb that refers to time.

For example:

That horse is very fast.

The truck quickly pulled out of the lot.

However, Americans often use fast as an adverb. This is rarely seen in British English. 

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