The process was lightning-fast from first interview to final loop. The recruiter was active and enthusiastic throughout the process, which started with a take-home 90-minute DSA problem, then a 60-minute system design loop, then a final loop with 2 "practical coding" problems, a system design problem, and a behavioral screen with the HM.
The practical coding focus of the final loop was nice, but unfortunately, it was clear that the first interviewer I had was not well-prepared. In general, he interrupted me constantly, tried to finish my thoughts for me (in a negative sense, i.e. misunderstanding/misinterpreting the clarification questions I was asking him), and being imprecise and inconsistent in his own instruction. But especially with the question itself, he kept pressing me to asymptotically improve the time complexity of an O(n*m) solution which could not be asymptotically improved in the worst case. I expressed this and pointed him to a number of iterative improvements, but he cut me off with every one I presented. At the end, I asked him what he was looking for, and he explained he wanted me to initialize an O(n*m) matrix, and then express that matrix as bitstrings. Bitstrings with n*m bits. So... yeah... He might've caught himself there even, because he concluded with a handwavey "and then you can do some bitwise operations I don't remember."
The rest of the loop went exceptionally well, so it was clear that this first interview is what killed my prospects, which left a sour taste in my mouth. I would recommend Amperity devote themselves to regular interview training refreshers for their employees, because this employee was a Lead Engineer who had been at the company for years, and while experience is great, it can lead to situations like these when one doesn't regularly dust off their hat before putting it on.