"Fast" is relative here. It's fast compared to Glacier and others, but it's going to be slower than the more expensive tiers.

You might not need to speculate much about how it works, it's probably implemented as described by Google themselves in slides 22ff. here:

http://www.pdsw.org/pdsw-discs17/slides/PDSW-DISCS-Google-Ke...

(In a nutshell, pack some hot data with a lot of cold data on many large drives, then put a Flash-based cache in front of them to get long tail performance predictability back.)