I have a strange performance problem with stunnel, could you help me?

Here is my setup:
PC1 & PC2 -- Switch (GbE) -- Switch (100mbps) -- Router -- modem -- Internet

PC1 has stunnel and HFS installed, sharing a folder to the internet.My internet upload speed is 20Mbps.

With this setup, I can serve files at 8Mbps over the internet, but if I disable stunnel the speed doubles to 16Mbps.
However, with stunnel active, if I try the same with PC2, I get 82Mbps (using the internet IP, it goes all the way to the router), so this does not appear to be a performance problem on the hardware.

Can you give me some hints on what is limiting my speed here?