server output bandwidth differs towards different locations

I have a VPS which has a 100 Mbps coection. My current coection is 50 Mbps to other sites, but with my vps I only get 2-3 Mbps, even though from other places the access is fast. What can cause this?

In detail: The vps in country A. It seems to work fine from country B, and country C city C1. But from country C city C2 I can access it only with a speed of 2-3 Mbps, doesn't matter if it's wget, ssh or openvpn.

It's pretty strange, do you have an idea of the cause, solution?