CO2 isn't toxic at CO levels and maybe not even performance impactful at 800 ppm, but there have been studies[1] that seem to indicate that even moderate levels of CO2 have measurable impacts.
My personal experience is that CO2 levels above 1000 ppm are easily noticeable once you can correlate how you feel with a measurable value.
[1] https://pubmed.ncbi.nlm.nih.gov/25117890/