promiscue mode - what does it let pass?

Hi
We have a strange phenomena. On an IPSec connection pings goes through fast. But if we open a webserver site on that line it last minutes the content arrives. With Wireshark we can see much packet loss. We played with the MTU, but this doesn’t change anything. But if the sniffer is enabled and the interface is in promiscue mode http traffic goes fast (but there are still packet loss)
What does promiscue mode do, except accepting any mac address?

Most of your packets go fast path, missing the IPSec tunnel. Make sure all your IPSec traffic does not go fast path.

Ok Thanks a lot - we will try it.