I’m using the Cisco AnyConnect Secure Mobility Client version 4.1 for connecting to a Cisco VPN. At some point, I noticed a significant speed drop. I don’t remember when it happened but I never got any speed higher than 50k bytes per second. That’s really slow compared to what my DSL allows.
The setup is pretty un-exciting: MacBook Pro ethernet via Thunderbolt display > Apple Time Capsule > GB switch > Fritzbox 3370 > DSL > Internet > Cisco Firewall.
After some further investigation today, a friend gave me a tip with regards to tweaking the MTU size. Turns out, Apple’s default of 1500 was actually the culprit. I didn’t expect that. My VPN is now working at full DSL speed.
Here is the link to testing and setting the MTU size: Fix WiFi on OS X by tweaking the MTU. Now, even thought the post talks about WiFi, it worked on my wired ethernet connection too.