It seems that the Internet is unhelpful at all. The original quote, where this simile appears, is:
“Once upon a time,” began Frank switching, quick as silk, to a
sonorous story-telling voice…
The full story may be checked on Google Books, from page 419 of Mammoth Deception (2013) by Aleta Whitaker.
Best Answer
I think the cited usage is something of a malapropism / mixed metaphor. It should be one of either...
Note that both the above expressions occur many times in Google Books (which I've linked to). But there are virtually no instances of quick as silk or smooth as a flash
Obviously there's often a difference between doing something quickly and doing it smoothly, but in the exact context they're much the same. It's possible the writer was being deliberately quirky by mixing up the two expressions (perhaps in order to force the reader to explicitly recognise both nuances). But I kinda doubt it - most likely it's just a "mistake".