Super fast hypercapnia...

[Shenji]

When in overpressured pure oxygen environment (3x+ normal pressure) I get hypercapnia very fast, about 1 and half minutes.
Is this intentional? So we can't skip the Eva suit?

[Joshu]

Hypercapnia is independent of oxygen levels. So it's kind of intentional!
1 and a half minutes is a bit fast, but you're able to survive mild and moderate hypercapnia just fine.
Eventually you should be able to find a scrubber on derelicts. Scrubbers will take the CO2 out of the air. Pressure suit should be rather usable if you run a larger ship with multiple rooms though. Venting atmo to space and re-pumping oxygen/nitrogen is one of the better strategies for surviving early game.

Overpressure is never going to work anymore even though it did in past versions.