In today's swim I did a few sets with bilateral breathing. As anticipated my pace was nothing like 1:44 min/100m as suggested from the last data. I don't know what happened there, but today I was around 2:10min/100m which is far more what I would expect. Inbetween a small proportion of the 66m intervals were at 2:00min/100m but I struggled to manage that.
I'm intrigued that as the session went on the bilateral pace was much like my 'fast' dominant side breathing. My chest felt tighter having to maintain the breath for 50% longer and I had perception of rushing but it didn't have much net effect on my pace. Perhaps there is a gain in swimming efficiency by keeping more streamlined which is offset by being closer to an aerobic threshold. This threshold might be centrally governed ('I must breath') rather than really physiological ('there isn't enough oxygen perfusion').