Cfxtrjtrk

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.

I think the cited usage is something of a malapropism / mixed metaphor. It should be one of either...

switching, [as] quick as a flash [to something else]

OR
switching, [as] smooth as silk [to something else]

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".

It is almost certainly a mistake, as the expression "quick as silk" not only makes no sense (silk has no speed) but it cannot be found anywhere else.

This is not strictly a "malapropism", as it is unlikely one would hear "flash" and think it was "silk". A malapropism is when someone gets an idiom wrong by substituting a similar sounding word, eg "he danced the flamingo" (instead of "flamenco").

Neither it is a "mixed metaphor", as this is a statement that contains two incompatible comparisons, for example "we need to iron out the bugs" (who irons bugs? It should be iron out the creases or crush the bugs).

I think what you have on your hands is a Malaphor - an incorrect mixture of two idioms, or clichés, also called an idiom blend.

If we assume the author didn't make a mistake here, then it feels to me like a reference to silk's "slipperiness" - imagine two pieces of silk cloth sliding over each other: fast and friction-less.

It may not be a common metaphor, but it makes perfect sense if you know what shot silk is - basically, it is silk fabric made with different colored warp and weft threads, so that every slight movement of the cloth changes the patterns of color you see.

Frank made a rapid shift from making a reference to a dirty joke about a princess, to retelling the story in elaborately "politically correct" language (she "sat contemplating ecological issues" etc).