There are some studies that already show negative effects on cognitive abilities at ~1,000 ppm levels (decision-making, etc.). Here's a study from Berkeley Lab:
I'm not aware of any more accurate studies. I'd also caution as with all things human studies: sample sizes are almost always too low. But the effects at 1,000 and 2,500 ppm appear to be significant. (I believe some of these results have already been taken into account in air quality regulation of highly critical positions such as air traffic control, or launch control at NASA.)
For example note how the 'taking initiative' metric is 'average' at 600 ppm already, and a significant sample size (~25% of participants) went into 'dysfunctional' performance at 2,500 ppm (if I'm reading their graph correctly). Note that it's very easy to reach 1,000+ or 2,000+ ppm levels even in ventilated spaces: the air at shopping malls in the winter can be consistently above 2,000 ppm.
(It's unclear to me whether their 600 ppm results are relative to 400 ppm results, or are the baseline the other results should be compared against.)
Personally I'd not be surprised if there was a measurable difference between ~280 ppm preindustrial levels and the current ~406 ppm levels, rising by almost 1% ppm every year:
So if you have a white collar job that requires high intellectual performance then having
really good air in your office is probably beneficial - and buy a CO₂ detector to get a picture of how good air quality is in the places you frequent - humans have no senses to detect such low levels of CO₂ concentration, which is an odorless gas. Being able to act quickly and think clearly in enclosed space is not something evolution ever forced humans to handle well.
Also note that CO₂ effects on brain performance appear to be entirely reversible - and there are perfectly safe health therapies for asthmatics that involve very high levels of CO₂. Obviously the negative cognitive effects of atmospheric CO₂ rise are permanent as long as we are breathing that air - but I'd not be surprised to see future whole-home (or in-car) ventilation systems filtering out CO₂ instead of using outside air - and keeping CO₂ levels permanently low.
The air used by the future Mars colony will also have an edge in terms of having lower CO₂ levels.