It absolutely is poorly ventilated.

I notice I'm not as mentally focused once it gets to 800 or so.

I have a CO2 monitor to keep it below 600 for productivity and concentration.

Remember, fresh air is around 420.

It's “less than ideal” level of ventilation, but it's also very far from “very poorly ventilated” (it's not even above the target level set by workplace regulation in my country, which is 600ppm above the baseline).