And what encryption is stunnel using? AES was picked to be fast so it may very well be outperforming whatever stunnel uses.

Can 1200 extra context switches and ~6400 extra interrupts per second use up 100% cpu? In fact you mention that for stunnel most of the CPU was in userspace and not the kernel which would indicate time spent actually using the CPU instead of doing context-switches and interrupts, which I assume top would show as "sys".

I also find the extra interrupts strange. I wonder if vCider is sending bigger packets and what caching/latency implications that might have.