I don’t think anyone avoiding talking about single threaded bottlenecks. The main one that matters is on sstable streaming during bootstrap, and the zero copy streaming was directly to address that. There’s not really any single threaded pieces in cassandra that matter beyond that? If anything the opposite is true - thread pools everywhere leading to context switching and data copying for no reason.
On a deeper note, it probably says something about the user base that people who don’t run it in prod think the perf is really bad and yet there are very few people submitting perf improvement patches. Maybe most of the power users aren’t that worried about perf because they either know how to tune a JVM or they happen to size their clusters based on bytes on disk?