Does a high level of CO2 harm or decrease your mental performance? Nope.
A 2019 study by researchers conclude that the brain's critical levels of cognition and decision making are not impaired when the human body is subjected to much higher levels of CO2 than found in Earth's atmosphere.
"It is well-established that rising levels of atmospheric CO2 pose no direct threat to human health...the ten U.S. researchers tested the mental acuity of 22 "astronaut-like" male and female subjects to four different CO2 concentrations (600, 1200, 2500 and 5000 ppm), subjecting them to standardized decision making and cognition tests...According to the authors, "there were no clear dose-response patterns for performance on either [the] Strategic Management Simulation or Cognition" evaluations...Consequently, when considering the full range of CO2 values examined, they conclude that "performance across tests did not differ between baseline (600 ppm) and the higher concentrations.""