I think it has more to do with the event driven nature of nginx vs the thread per connection nature of Apache.
If you're looking for this, you want kTLS, plus or minus nic accelerated kTLS. Read the Netflix CDN presentiatons for the state of the art, and see what you can make work with your system, etc.
I know abut Netflix work, and even some of this work trickled down to base FreeBSD (and opensource), but is still very fragile, not completely open-sourced and require very special hardware (1 or 2 models of specific NICs with specific versions of firmware) to work.
TLS kills this, alas, and, I think, with TLS OpenSSL will be bottleneck for static content, not server.