The quote heading this post - "children with both ASD [autism spectrum disorder] and subclinical autistic traits have lower neuropsychological performance" - comes from the findings reported by Fjola Hyseni and colleagues .
Researchers report results from a study "embedded in the Generation R Study" (an initiative that has appeared before on this blog) examining whether neuropsychological performance as measured by "using subtests of the NEPSY-II-NL" showed any connection to autistic signs and symptoms. The sorts of things looked at via the NEPSY-II included "attention and executive functioning, memory and learning, sensorimotor, language, and the visuospatial domain."
Actually, the Hyseni study was a multi-factorial study in that their analysis "involved an ASD case versus control analysis comparing children with a diagnosis of ASD to children without an ASD diagnosis" and also "to test for a linear relationship between autistic traits and NEPSY-II performance in the entire sample." Finally, they "excluded both children with a diagnosis of ASD and children with high autistic symptoms" but still looked at scores on the Social Responsiveness Scale (SRS) as a function of NEPSY-II test performance.
Results: based on the inclusion of data from over 1000 children, there were some important details attached to the authors observation that autistic traits were negatively correlated with neuropsychological performance. So, as well as higher levels of autistic traits or a diagnosis of autism being associated with lower neuropsychological performance across various domains and taking into account various other potential confounding variables - covariates - (gender, age, ethnicity, child attention problems, etc), so those analyses of SRS scores of participants without a diagnosis of autism or high autistic traits yielded important findings. For the 960 children who this covered, authors reported that "the significant relationship between SRS scores and neuropsychological performance remained", at least when it came to memory and learning, sensorimotor functioning, and language domains following adjustment for those covariates. The authors conclude that: "Our results provide support for a relationship along the continuum between autistic symptoms and neuropsychological performance in school-age children who encompass children with subclinical autistic traits." Further investigations are implied in this area on the basis for example, that: "the measurements of autistic traits, neuropsychological functioning, and IQ were not performed at the same time; thus, there was a short time difference between the two measurements."
There are some important implications from such results. Not least, as the authors point out: "children with both ASD and subclinical autistic traits have lower neuropsychological performance" and also that they: "may provide an understanding of why some children without an ASD diagnosis may require some additional assistance within academic settings." I hasten to add that neuropsychological performance is not necessarily the same as academic performance, and so one has to be quite careful of where such discussions could lead. I also think back to those ideas that have perpetuated autism down the ages suggesting that a diagnosis of autism confers some sort of 'savant' advantage to the masses. I've talked before about how sweeping a generalisation such thinking is (see here) and indeed, the Hyseni findings kinda add further cold water to such universal sentiments. People diagnosed as on the autism spectrum are gifted in many ways, but not necessarily because of any diagnostic label they have received and not necessarily consistently so ...
And on the topic of autistic traits potentially 'influencing' performance, the findings reported by Crehan and colleagues , including one John Constantino - "the authors of the SRS" - are also pertinent. To quote: "Splitting the SRS scores into three severity classes revealed that impaired social responsiveness is significantly related to competency." One also has to wonder in the longer term, whether the sentiments expressed by Skylark & Baron-Cohen  on how certain autistic traits (particularly socially implicated traits) might "influence a person’s financial circumstances" could also 'interact' with the findings being discussed.
Much food for thought.
 Hyseni F. et al. Autistic traits and neuropsychological performance in 6- to-10-year-old children: a population-based study. Child Neuropsychol. 2018 Apr 23:1-18.
 Lindor E. et al. Superior Visual Search and Crowding Abilities Are Not Characteristic of All Individuals on the Autism Spectrum. J Autism Dev Disord. 2018. May 22.
 Crehan ET. et al. Tracking the Influence of Autistic Traits on Competencies Among School Aged Children with Subthreshold Autistic Traits: A Longitudinal Study. Child Psychiatry Hum Dev. 2018 May 11.
 Skylark WJ. & Baron-Cohen S. Initial evidence that non-clinical autistic traits are associated with lower income. Mol Autism. 2017 Nov 13;8:61.