In direct speech, should there be a comma after the clause but before 'as'?

Although this is stylistic, the common guidance can be exemplified by "When to Use a Comma Before 'As'" at Magnum Proofreading Services.

It starts off with an example that doesn't match the one used in the question:

George cleaned the house as his wife had asked him to.

George cleaned the house, as his wife had asked him to.

Both of these examples are grammatical. However, they do not mean the same thing, and so it is important to know how the use of a comma changes the meaning of the sentence.

Adding the comma did not change the meaning of any words in the sentence except for one: as. In sentences with the structure of our example, if there is no comma before as, then as means “in the way that” or “while.” When you insert a comma before as, its meaning changes to “because.”

So, the first sentence above, which contains no comma, can be rephrased as “George cleaned the house in the way that his wife had asked him to.”

The second sentence, which contains a comma, can be rephrased as “George cleaned the house because his wife had asked him to.”

I suspect most people, if looking for a difference, would consider this a reasonable interpretation of the difference between the two sentences. I also interpret it in this way, but that's only my personal interpretation.

However, this is not necessarily the interpretation that everybody would make. For those who see commas as nothing more than just a pause, the comma here might be a useful tool to help signal a change in meaning, but not be necessary for that change in meaning. In such an interpretation, the meaning is derived from context rather than the punctuation itself. It also means that there is a degree of ambiguity in the sentence itself, with or without the comma.

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The article then provides an example that does match the one used in this question:

Note that, for certain sentences, there is only one correct decision regarding the use of a comma based on the context.

George cleaned the house as he listened to the radio.

George cleaned the house, as he listened to the radio.

Putting a comma before as in this sentence is a mistake. George clearly cleaned the house while he listened to the radio, not because he was listening to the radio. The second sentence is still grammatical, but it isn’t logical.

Although the grammar has changed slightly, the same analysis of comma use has been applied. If the analysis of the first example is accepted, it's easy to see why it would be applied to the second on grounds of consistency. Again, however, if the first analysis is questioned, it's also easy enough to question the second analysis.

Note, too, that it might be erroneous to say that George's cleaning of the house could not logically be caused by his radio listening. That certainly is an unusual thing to say, but George could have some kind of Pavlovian reaction to radios that does indeed cause him to clean whenever he hears them …

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Note what this interpretation does to the example sentences in the question:

• "Wow!" gasped Sarah as she spotted the sparkling fairy from all John's stories.
  She gasped while she spotted the fairy.

• "Wow!" gasped Sarah, as she spotted the sparkling fairy from all John's stories.
  She gasped because she spotted the fairy.

Even if you accept this interpretation, it makes sense that she would gasp because she saw the fairy. So, there is no mistake here at all, merely two different meanings—and the question to be answered is which meaning you want to convey.

Note that even with this interpretation, the comma still can be used without conveying the because meaning, if the sentence is rephrased slightly:

"Wow!" gasped Sarah, spotting the sparkling fairy from all John's stories.

Removing the as and changing she spotted to just spotting both maintains the pause and dispenses with any possible syntactic ambiguity.