Our construction performance specification is 30 pages long, but also contains many descriptions and, for example, a picture of the stairs. If you summarize it briefly and concisely, it would certainly also fit on 15 pages. Here quality clearly takes precedence over quantity.
I would just read the construction performance specification in detail and really question every point mentally. If something is unclear to you, then I would write it down and discuss it with the general contractor. And then you can still decide what else should be included and what not. Additionally, think about what is in the plans and what might still be missing in the construction performance specification. For example, in our case the porch was in the drawings but not a single sentence in the construction performance specification. So we had the execution, the material, etc. added there.
In case of doubt, there is the option to have the construction performance specification checked by an expert. They can then tell you exactly what, from their point of view, is good, what should be changed, and what is still missing. We once asked one, who wanted about 400 euros for checking the construction performance specification – just as a rough guideline.