[0xfaded]

> the sys design interview was about an obscure optimization problem that I specialized in for years - though I didn't let on that fact

Honestly I feel this situation is an invitation/opportunity to show your depth. I had a similar experience where I was asked to implement a box filter, which I did naively, and then asked to do it the clever way.

I remembered about Haar classifiers https://docs.opencv.org/3.4/db/d28/tutorial_cascade_classifi... which solve the same problem, and I was familiar with the OpenCV implementation. I mentioned this and started to write code, but at that point the interviewer was much more interested about why I knew about some obscure old-school opencv method than finishing the coding exercise.