it's not just cutting speed that's a problem. Edge quality will drop off too. Piercing will be less effective and increase the chance of spatter fouling the nozzle and lens. I have operated mild steel O2 cutting where there has been accidental contamination of the O2 due to a faulty solenoid valve in another machine on the same O2 circuit. The contamination occurred elsewhere in the factory, and the air managed to work it's way back towards the O2 source and mess up my cut. I have no idea what % contamination this would work out to be but given the circumstances I expect it would have been only a small amount.