Subtle technique (not fitness) worth at least 5 sec/100yds

I just noticed that when I do intervals breathing to my right, I swim at 1:30 pace. When I breath to my left however, it's 1:37 pace. Same effort - in fact, I have to really go near all-out with left-breathing to equal my T-pace right breathing.

Took me awhile to realize this since I breathe one-sided alternating on each length of the pool and I breath on every stroke cycle, so I didn't realize there was such a big gap in my pacing until I had a 'pacer' in an adjacent lane my speed a few weeks ago who I was yo-yoing with each length to my surprise. I've also been swimming this wayfor the past 2 years (adult onset swimmer) so both left and right sided breathing have been getting equal volume/intensity work.

It's taken me a week to figure out what the difference is, and while I'm still not able to exactly identify the problem causing the speed difference,  think that a lot of it is a very mild bobbing that occurs when I breath on my left. While it feels smooth when I swim, it's really brought out when I do a one-armed stroke with ankles banded - no problem and smooth on the right, but choppy and irregular on the left.

Just wanted to throw this out there as to how a very subtle swim technique difference can yield a big time difference. I thought this was a particularly good example since my swim 'strength' is equal for both. I'll also add my 2cents that the ankle banded swimming, both 2-armed and one-armed really brings out imbalances in body position and stroke, and is worth a try. Basically, if you're struggling to finish laps with the band on, even with 1 arm strokes, you've got plenty of room to improve, and at least 5 sec/100yds to gain as per my stroke difference.