Personally, I don't agree on the word "performance" -- as well as many others --
being classified by dictionaries as countable or uncountable.
According to grammar books, uncountable, or non-count, nouns are those which will not take the indefinite article "a/n" and will take, instead, "some" or some partitive (a piece of, an item of, etc.). That is clearly not the case with "performance":
- That has been an outstanding performance.
My impression has always been that dictionaries tend to confuse countable-uncountable with concrete-abstract. It is a fact that abstract nouns, which generally derive from verbs and adjectives, do not usually take the plural form, that is, unless we refer to instances of the concept conveyed by the noun.
P. E. Dant's example in his comment above provides us with an excellent example of this:
In a sentence like "Her performance is superb," the noun could be uncountable (in reference to her job skills, for instance) or countable (in reference to her reading of a part in a theatrical production).
My feeling is that, when we refer to her acting skills in general, we are using the abstract "performance", while the sense of the noun becomes concrete when reference is made to her specific acting on a particular occasion.
When used in a concrete sense, the noun will enable pluralization (different instances of performance), while it will be used in the singular form when used in a more abstract sense.
The fact that dictionaries usually confuse "abstract" with "uncountable" may stem from the considerable overlap between abstract and non-count, as recognized by Quirk on page 246 of his Comprehensive Grammar of the English Language:
My claim is that the fact that an abstract noun is generally used in the singular does not make it uncountable.
In reply to BillJ's objection, please find below how Quirk associates, on page 247 of his book mentioned above, the use of articles with the classification into count and non-count nouns: