In the [FAQs](http://aws.amazon.com/simpledb/faqs/) of AWS SimpleDb service I have noticed paragraph

> 
> Q: When should I use Amazon S3 vs. Amazon SimpleDB?
> 
> 
> Amazon S3 stores raw data. Amazon SimpleDB takes your data as input
>  and indexes all the attributes, enabling you to quickly query that
>  data. Additionally, Amazon S3 and Amazon SimpleDB use different types
>  of physical storage. Amazon S3 uses dense storage drives that are
>  optimized for storing larger objects inexpensively. Amazon SimpleDB
>  stores smaller bits of data and uses less dense drives that are
>  optimized for data access speed.
> 
> 
> 

Can somebody explain how **AWS SimpleDB** reach high data access speed with using of **less dense drives**?

As I know: more density -> more speed.